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%%%
%%%\title[Expressing the statement of Feit-Thomson theorem
%%%with diagrams in the category of finite groups
%%%\ \ \ \ \ \ \ \ \ \ \ \ \ \ {\textbf{\tiny To} {\textsf{\bf Grigori Mints Z"L}} \tiny In memoriam}
%%%]{Expressing the statement of Feit-Thomson theorem
%%%with diagrams in the category of finite groups
%%%\\{\tiny.}
%%%\\
%%%{{{\tiny To} {\textsf{Grigori Mints Z"L}} \tiny In memoriam}}
%%%}
%%%\author[Misha Gavrilovich]{Misha Gavrilovich\ \ \ \ \ \
%%%{{{\tiny } {\text{July 2016}}}}}
%%%\date{2013}
%%%
%\extraline{\copyright\ 2014 2015 Misha Gavrilovich {\tt mi\!\!\!ishap\!\!\!p@sd\!\!\!df.org http://mishap.sdf.org/mints-lifting-property-as-negation}}
%\classno{97A80, 97B70}
%\journal{ mishap.sdf.org/mints \hfill
%\hfill ISSN 2053-1451}
%\volume{5}
%\parskip 5pt
%\setcounter{page}{23}
%\maketitle
%\setlength{\epigraphwidth}{0.5\textwidth}
%\epigraph{\normalsize There's no point in being grown up if you can't be childish sometimes
%}{ {\emph{ %fixme: a better epigraph is needed.
%}}
%}
\begin{document}
\date{}%9 May 2017}
\title{
%%%%n
A diagram chasing formalisation of elementary topological properties
%The unreasonable power of the lifting property in elementary mathematics
}
\author{
%%%%n
misha gavrilovich\thanks{A draft; comments welcome.
$\mathtt{mi\!\!\!ishap\!\!\!p@sd\!\!\!df.org}$.
This draft is essentially a part of a more verbose draft on the expressive power of the lifting property available at http://mishap.sdf.org/expressive-power-of-the-lifting-property.pdf.
}\\
{\small\tt in memoriam: evgenii shurygin}
}
\maketitle
%%epigraph:
%\setlength{\epigraphwidth}{0.8\textwidth}
%\epigraph
\vskip-6pt{%In the following sections we bring up
\small.\hskip-18pt instances of human and animal behavior
%%%% which are, on one hand,
[...] miraculously complicated,
%%%%on the other hand
%%%%[...] they have
[of] little, if any, pragmatic (survival/reproduction) value, %.
%%%% From this we conclude [...]
%%%% %that since the corresponding features of ergobrains were not the primarily targets
%%%% %pecifically selected for by the evolution,
%%%[...] they
are due to internal constraints on
possible architectures of unknown to us functional ``mental structures". \\
{\tiny.}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {\small Misha Gromov, Structures, Learning and Ergosystems.}
%%%}{
%%% Gromov, Ergobrain
}
%%%
\begin{abstract}
%%%%+ abstract:
We introduce a simple formal syntax %and semantics
and use it to rewrite
in a concise, uniform and intuitive way several %a number of
standard definitions in topology which are usually expressed in words.
The definitions include compact, discrete, connected, and
totally disconnected spaces, dense image, induced topology, closed subsets, and
some of the separation axioms. The syntax is of category-theoretic flavour and is based
on the observation that these properties
can all be defined category-theoretically by repeated application of a standard
category theory trick, the Quillen lifting property (orthogonality of morphisms),
starting from a single (counter)example.
We hope our reformulations may be use in formalisation of elementary topology and in teaching.
%%%These expressions are based on the simplest (counter)example and in this sence are intuitive,
%%%and, arguably, are short enough (several bytes) to be found by a brute force search
%%%and may be of use in an automatic theorem prover.
\end{abstract}
\section{
%%%%
Introduction.
}
{\em Generalities. Motivation.}
We introduce a simple formal syntax %and semantics
and use it to rewrite
in a concise, uniform and intuitive way several %a number of
standard definitions in topology which are usually expressed in words.
The definitions include compact, discrete, connected, and
totally disconnected spaces, dense image, induced topology, closed subsets, and
some of the separation axioms.
The formal expressions are uniform and intuitive in the sense that
each is based on the simplest (counter)example
and treats orthogonality of morphisms as negation, i.e.~as a way to avoid counterexamples. % and in this sense are intuitive,
Arguably, they are short enough (several bytes) that a brute force algorithm can produce
in practice a short list containing them all.
We hope this syntax may be of use in automatic theorem proving and in teaching elementary topology,
and, if developed further, may allow to rewrite the traditional definitions
in elementary topology into a new presentation based on diagram chasing
that is easier for both humans and machines to understand.
In a somewhat similar way the diagram chasing presentation\footnote{
Someone interested in implementation may want to a look at a sample Python code [BaysQuilder, NOTES]
and the appendix to [Gavrilovich, A homotopy approach to set theory, pp.13-16]
to get some idea on how to view the axioms of a model category as diagram chasing rules.}
based on the axioms and
formalism of a model category allows to rewrite some definitions of elementary homotopy theory.
One cannot but wonder whether these elementary observations may provide a useful point of view
on tame topology of Grothendieck, a foundation of topology ``without false
problems" and ``wild phenomena" ``at the very beginning". We also note that in a sense,
our expressions are implicitly present in [Bourbaki, General Topology] but are described there in words.
The syntax is a very simple combinatorial formalism of category theoretic flavour
based on %diagram chasing with
finite preorders, or, equivalently,
finite categories of certain kind.
The semantics is based on viewing finite topological spaces as preorders or categories, and
the Quillen lifting property (orthogonality
of morphisms) in the category of topological spaces.
The intuition (cf.~[Gavrilovich, DMG]) is based on the following:
(a) the expressions are based on simple counterexamples, often a map between a point and two points
(a) treating the lifting property as negation
(c) correlations between the diagrams and the text of the definitions.
{\em Illustrative examples.}
As an illustration, let us list formal expressions
for: compactness; surjective, subset, discrete, split; connected, injective;
induced topology, separation axiom $T_1$;
dense image, closed subset
\begin{verbatim}
{ {a}-->{a->b} }^r_{<5}^lr compactness
{ {}-->{a} }^r surjective
{ {}-->{a} }^rr subset
{ {}-->{a} }^rl discrete
{ {}-->{a} }^lrl split morphisms
{ {a,b}-->{a=b} }^l connected
{ {a,b}-->{a=b} }^r injective
{ {a->b}-->{a=b} }^r induced topology
{ {a->b}-->{a=b} }^l separation axiom T1
{ {b}-->{a->b} }^l dense image
{ {b}-->{a->b} }^lr closed subset
\end{verbatim}
Moreover, these expressions are straightforward to find
once you note that they are based on the simplest counterexamples
and treat orthogonality/lifting property as negation,
e.g.~{\tt \{a\}-->\{a->b\} } %{a}-->{a->b} is
denotes the simplest non-proper map sending a point into the open
point of the space {\tt \{a->b\} } with open point {\tt a} and closed point {\tt b};
``negated" three times, this counterexample gives the notion of a proper map.
%???? EXPLAIN SIMPLEST COUNTEREXMLE SYNTACTICALLY!!!
Now let us explain how to evaluate the expression % {\tt \{ \{a\}-->\{a->b\} \}\^ r\_ \{<5\}\^lr }
for compactness in the category of topological spaces:
first take (a class consisting of) the single non-proper map {\tt \{a\}-->\{a->b\} },
the inclusion of the open point into the Sierpinski space,
then take the subclass of its right orthogonal
consisting of the maps between spaces each with less than 5 points
(the subclass happens to be the class of all proper, equiv. closed, maps
between such spaces), and then finally take its left, then right orthogonal.
An easy argument (cf.~the proof of Claim~2) shows you thus obtain a subclass of the class of proper maps
which contains all proper maps between finite spaces
and all proper maps between normal (T4) spaces. We do not know
if it is the class of proper maps (cf.~Conjecture~1 below).
% which concisely expresses a number of
% elementary topological properties, e.g. compactness, connectedness, dense image, separation axioms.
% These expressions are based on the simplest (counter)example and in this sense are intuitive,
% and, arguably, are short enough (several bytes) to be found by a brute force search.
% We hope this may be of use in automatic theorem proving and in teaching elementary topology.
% %by an automatic theorem prover able to work with diagram chasing arguments.
%
Our observations indicate that certain classical definitions and theorems of topology,
such as Tychonoff theorem and implications between separation axioms, %?? a Hausdorff compact being normal,
can be understood in a purely finitary way, as combinatorial rules of diagram chasing flavour.
% The syntax is based on finite preorders, or, equivalently but more conceptually,
% finite categories such that there is at most a single morphisms
% from any object to any object; the semantics is based on finite topological spaces and the Quillen lifting property (orthogonality
% of morphisms) in the category of topological spaces and finite topological spaces.
{\em Key observation} %
%
The syntax is based on the following observation, which applies not only to topology:
\begin{quote}
a number of elementary properties from a first-year course
can be defined category-theoretically by
repeated application of a standard category theory trick,
the Quillen lifting property,
starting from a class of explicitly given morphisms,
often consisting of a single (counter)example
\end{quote}
In this short note we discuss only examples of such properties in topology;
they include the notions of: compact, discrete, connected, and
totally disconnected spaces, dense image, induced topology, closed subsets, and separation axioms.
See [Gavrilovich, Lifting Property] for other examples. They include
complete metric spaces, a subset of a metric space being closed.
Examples in algebra include: finite groups being
nilpotent, solvable, torsion-free, $p$-groups, and prime-to-$p$ groups;
injective and projective
modules; injective, surjective, and split homomorphisms.
These examples and the observation above
are elementary, yet we were unable to find them in literature.
{\em Structure of the paper.}
%A mathematically inclined reader might want to read
%only the first two sections with miscellaneous examples of lifting properties
%and a combinatorial notation for elementary properties of topological spaces.
%A logician might want to read in the third section our suggestions towards
%a theorem prover/proof system for elementary topology based on diagram
%chasing. Appendix~A states separation axioms in terms of lifting properties
%and finite topological spaces. Appendix~B reproduces some references we use.
%
%Finally, the last section attempts to explain our motivation and says
%a few words about the concept of ergostructure by Misha Gromov.
%
%We would also like to draw attention to Conjecture~1 (a charaterisation of the class of proper maps)
%and Question~2 asking for a characterisation of the circle and the interval.
The goal of this short note is a concise exposition of the examples in topology
and directly related open questions.
A reader interested in a broader context or a more verbose exposition
might want to look at a draft [Gavrilovich, Lifting Property].
Further examples and speculations may be found in drafts [Gavrilovich, Lifting Property], [Gavrilovich, Tame topology].
There we offer detailed speculations on how to rewrite elementary topology in terms of diagram chasing rules
and labelled finite categories.
{\em History.}
%observation which appears not well-known:
%\begin{quote}
% a number of elementary properties from a first-year course
% can be defined category-theoretically by
% repeated application of a standard category theory trick,
% the Quillen lifting property,
% starting from a class of explicitly given morphisms,
% often consisting of a single (counter)example
%\end{quote}
%
%In particular, several elementary notions of topology
%can be defined purely formally by very short expressions (several bytes)
%in a natural category-theoretic formalism (explained below),
%e.g. \begin{quote}
% compactness is "$((\{a\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr}$", \\
% connectedness is "$(\{a,b\}\longrightarrow \{a=b\})^l$", \\
% dense image is "$(\{b\}\longrightarrow \{a{\small\searrow}b\})^l$". \\
% \end{quote}
%
%We suggest it appears worthwhile to try to develop a formalism,
%or rather a very short program (kilobytes) based on such a formalism,
%which supports reasoning in elementary topology.
%
These observations arose in an attempt to understand ideas of
Misha Gromov [Memorandum Ergo] about ergologic/ergostructure/ergosystems.
Oversimplifying, ergologic is a kind of reasoning which helps to
understand how to
generate proper concepts, ask interesting questions, and, more generally,
produce interesting rather than useful or correct behaviour.
He conjectures there is a related class of
mathematical, essentially combinatorial, structures,
called {\em ergostructures} or {\em ergosystems},
and that this concept might eventually
help to understand complex biological behaviour including learning
and create mathematically interesting models of these processes.
We hope our observations may eventually help to uncover
an essentially combinatorial reasoning behind elementary topology,
and thereby suggest an example of an ergostructure.
From this point of view, our examples suggest a short algorithm
to generate a number
of interesting notions in topology.
{\em Related works.}
This paper continues work started in [DMG], a rather leisurely
introduction to some of the ideas presented here
here which aims to express the intuition behind
our syntax.
Draft [Gavrilovich, Elementary Topology] shows how to view several
topology notions and arguments in [Bourbaki, General Topology]
as diagram chasing calculations with finite categories.
Draft [Gavrilovich, Tame Topology] is more speculative but less verbose;
it has several more examples dealing with compactness,
in particular it shows that a number of consequences of compactness
can be expressed as a change of order of quantifiers in a formula.
Notably, these drafts show
how to ``read off" a simplicial topological space
from the definition of a uniform space, see also Remark 7 and Conjecture 2
%\ref{sTop:1}~and~\ref{simp:obj:unifrom}
in [Gavrilovich, Lifting property].
%{\em Structure of the paper.} A mathematically inclined reader might want to read
%only the first two sections with miscellaneous examples of lifting properties
%and a combinatorial notation for elementary properties of topological spaces.
%A logician might want to read in the third section our suggestions towards
%a theorem prover/proof system for elementary topology based on diagram
%chasing. Appendix~A states separation axioms in terms of lifting properties
%and finite topological spaces. Appendix~B reproduces some references we use.
%
%Finally, the last section attempts to explain our motivation and says
%a few words about the concept of ergostructure by Misha Gromov.
%
%We would also like to draw attention to Conjecture~1 (a charaterisation of the class of proper maps)
%and Question~2 asking for a characterisation of the circle and the interval.
\section{Expressions for topological properties}
\subsection{
%%%%n
The lifting property: the key observation
}
For a property \ensuremath{C} of arrows (morphisms) in a category, define
$$ C^l := \{ \ensuremath{f} :\text{ for each }g \in C\ \ensuremath{f} \,\rightthreetimes\, \ensuremath{g} \} $$
$$ C^r := \{ \ensuremath{g} :\text{ for each }f \in C\ \ensuremath{f} \,\rightthreetimes\, \ensuremath{g} \} $$
$$ C^{lr}:=(C^l)^r, ... $$
here $f \,\rightthreetimes\, g$ reads `` $f$ has the left lifting property wrt $g$ ",
`` $f$ is (left) orthogonal to $g$ ",
i.e. for $f:A\longrightarrow B$, $g:X\longrightarrow Y$,
$f \,\rightthreetimes\, g$ iff for each $i:A\longrightarrow X$, $j:B\longrightarrow Y$ such that $ig=fj$ (``the square commutes"),
there is $j':B\longrightarrow X$ such that $fj'=i$ and $j'g=j$ (``there is a diagonal
making the diagram commute").
The following observation is enough to reconstruct all the examples in this
paper, with a bit of search and computation.
\begin{quote}
{\bf Observation.}\\
A number of elementary properties can be obtained by repeatedly passing
to the left or right orthogonal $C^l, C^r, C^{lr}, C^{ll}, C^{rl}, C^{rr},...$
starting from a simple class of morphisms, often
a single (counter)example to the property you define.
\end{quote}
A useful intuition is to think that the property of left-lifting against a
class \ensuremath{C} is a kind of negation of the property of being in \ensuremath{C}, and that
right-lifting is another kind of negation. Hence the classes obtained from C
by taking orthogonals an odd number of times, such as $C^l, C^r, C^{lrl}, C^{lll}$
etc., represent various kinds of negation of \ensuremath{C}, so $C^l, C^r, C^{lrl}, C^{lll}$ each
consists of morphisms which are far from having property C.
Taking the orthogonal of a class \ensuremath{C} is a simple way to define a class of
morphisms excluding non-isomorphisms from \ensuremath{C}, in a way which is useful in a
diagram chasing computation.
The class $C^l$ is always closed under retracts, pullbacks, (small) products
(whenever they exist in the category) and composition of morphisms, and
contains all isomorphisms of C. Meanwhile, $C^r$ is closed under retracts,
pushouts, (small) coproducts and transfinite composition (filtered colimits) of
morphisms (whenever they exist in the category), and also contains all
isomorphisms.
For example, the notion of isomorphism can be obtained starting from the class
of all morphisms, or any single example of an isomorphism:
$$
(Isomorphisms) = (all\ morphisms)^l = (all\ morphisms)^r = (h)^{lr} = (h)^{rl}
$$
where \ensuremath{h } is an arbitrary isomorphism.
%\input lifting_property_examples_figure.tex
\def\rrt#1#2#3#4#5#6{\xymatrix{ {#1} \ar[r]^{} \ar@{->}[d]_{#2} & {#4} \ar[d]^{#5} \\ {#3} \ar[r] \ar@{-->}[ur]^{}& {#6} }}
\begin{figure}
\begin{center}
\large
$ (a)\ \xymatrix{ A \ar[r]^{i} \ar@{->}[d]_f & X \ar[d]^g \\ B \ar[r]_-{j} \ar@{-->}[ur]^{{\tilde j}}& Y }$% \
%$\rrt ABXY$\ \ \ \
$(b)\ \rrt {\{\}} {} {\{\bullet\}} X {\therefore(surj)} Y $%\
$(c)\ \rrt {\{\bullet,\bullet\}} {} {\{\bullet\}} X {\therefore(inj)} Y $%\ \
%$(d)\ \rrt X {\therefore(inj)} {Y} {\{x,y\}} {} {\{x=y\}}$\
\end{center}
\caption{\label{fig1}\normalsize
Lifting properties. %Dots $\therefore$ indicate free variables and what property of these variables is being defined;
%, i.e.~a property of what is being defined and how is it to be labelled
%in a diagram chasing calculation, ``$\therefore(surj)$" reads as:
%given a (valid) diagram, add label $(surj)$ to the corresponding arrow.\newline
(a) The definition of a lifting property $f\rtt g$.
%: for each $i:A\lra X$ and $j:B\lra Y$
%making the square commutative, i.e.~$f\circ j=i\circ g$, there is a diagonal arrow $\tilde j:B\lra X$ making the total diagram
%$A\xra f B\xra {\tilde j} X\xra g Y, A\xra i X, B\xra j Y$ commutative, i.e.~$f\circ \tilde j=i$ and $\tilde j\circ g=j$.
(b) $X\lra Y$ is surjective %\newline
(c) $X\lra Y$ is injective
%; $X\lra Y$ is an epicmorphism if we forget %never use
%that $\{\bullet\}$ denotes a singleton (rather than an arbitrary object
%and thus $\{\bullet,\bullet\}\lra\{\bullet\}$ denotes an arbitrary morphism $Z\sqcup Z\xra{(id,id)} Z$)\newline
% (d) $X\lra Y$ is injective, in the category of Sets; $\pi_0(X)\lra\pi_0(Y)$ is injective if
% the diagram is interpreted in the category
%of topological spaces.
}
\end{figure}
{\bf
Example.} %\begin{quote}
Take $C=\{ \emptyset\longrightarrow \{*\} \}$ in Sets and Top.
Let us show that $C^l$ is the class of surjections, $C^{rr}$ is the class of subsets,
$C^l$ consists of maps $f:A\longrightarrow B$ such that either $A=B=\emptyset$
or $A\neq \emptyset$, \ensuremath{B} arbitrary. Further,
in Sets, $C^{rl}$ is the class of injections, and in Top, $C^{rl}$
is the class of maps of form $A\longrightarrow A\cup D$, $D$ is discrete;
both in Sets and Top, $C^{lr}$
is the class of maps $A\longrightarrow B$ such that either $A=\emptyset$ or the map is an isomorphism.
% $A\longrightarrow \ensuremath{B} \,\rightthreetimes\, \emptyset\longrightarrow \{*\}$ iff $A=B=\emptyset$ or $A\neq \emptyset$, \ensuremath{B} arbitrary. Indeed,
% if $A\neq \emptyset$, there is no map $i:A\longrightarrow X=\emptyset$ and the lifting property
% holds vacuously; if $A=\emptyset\neq B$, there exist unique maps $i:A=\emptyset\longrightarrow X=\emptyset$,
% $j:B\longrightarrow Y=\{*\}$, but no map $j':B\longrightarrow X=\emptyset$ as $B\neq \emptyset$ by assumption.
%
% $\emptyset\longrightarrow \{*\} \,\rightthreetimes\, g$ iff $g:X\longrightarrow Y$ is surjective; indeed,
% the map $j:B=\{*\}\longrightarrow Y$ picks a point in $Y$ and $j':B=\{*\}\longrightarrow X$
% picks its preimage as $j'g=j$; the other condition $fj'=i:\emptyset\longrightarrow X$
% holds trivially. Thus $(\emptyset\longrightarrow \{*\})^r$ is the class of surjections.
%
% In Sets, $(\emptyset\longrightarrow \{*\})^{rl}$ is the class of injections, i.e.
% $f \,\rightthreetimes\, g$ for each surjection \ensuremath{g} iff \ensuremath{f} is injective; indeed,
% for such \ensuremath{f} and \ensuremath{g} the following is well-defined:
% set $j'(b)=i(f^{ -1}(b))$ for \ensuremath{b} in $Im f$, and $j'(b)=g^{ -1}(j(b))$
% otherwise; for injective \ensuremath{f} $j'(b)$ does not depend on the choice
% of a preimage of b, and for \ensuremath{g} surjective a preimage always exists.
%
% In Top, $(\emptyset\longrightarrow \{0\})^{rl}$ is the class of maps of form $A\longrightarrow A\cup D$, $D$ is discrete;
% given a map $A\longrightarrow B$, consider $A\longrightarrow \ensuremath{B} \,\rightthreetimes\, ImA\cup (B\setminus A)\longrightarrow B$ where $ImA\cup D\longrightarrow B$
% denotes the disjoint union of the image of A in \ensuremath{B} with induced topology,
% and $B\setminus A$ equipped with the discrete topology.
%
In both Sets and Top, $(\emptyset\longrightarrow \{*\})^{rr}$ is the class of subsets, i.e.
injective maps $A\hookrightarrow B$ where the topology on $A$ is induced from $B$.
%\end{quote}
\subsection{
Notation for maps of finite topological spaces
}
Now we %First we must
introduce notation for maps of finite topological spaces we use.
%Two features are important for us:
%\begin{itemize}
%\item[ 1.] it reminds one that a finite topological space is a category
% (degenerate if you like)
%\item[ 2.] it does not allow one to talk conveniently about non-identity
% endomorphisms of finite topological spaces. We hope this may help define
% a decidable fragment of elementary topology because there is
% a decidable fragment of diagram chasing without endomorphisms, see [GLZ].
%\end{itemize}
%
A topological space comes with a {\em specialisation preorder} on its points: for
points $x,y \in X$, $x \leq y$ iff $y \in cl x$ , or equivalently, a category whose
objects are points of \ensuremath{X} and there is a unique morphism $x{\small\searrow}y$ iff $y \in cl x$.
For a finite topological space X, the specialisation preorder or equivalently
the category uniquely determines the space: a subset of \ensuremath{X} is closed iff it is
downward closed, or equivalently, there are no morphisms going outside the
The monotone maps (i.e.~functors) are the continuous maps for this topology.
We denote a finite topological space by a list of the arrows (morphisms) in
the corresponding category; '$\leftrightarrow $' denotes an isomorphism and '$=$' denotes
the identity morphism; the empty list is denoted by either $\emptyset$ or ${\tt \{\}}$.
An arrow between two such lists denotes
a continuous map (a functor) which sends each point to the correspondingly
labelled point, but possibly turning some morphisms into identity morphisms,
thus gluing together some points.
Thus, each point goes to ``itself" and $$
\{a,b\}\longrightarrow \{a{\small\searrow}b\}\longrightarrow \{a\leftrightarrow b\}\longrightarrow \{a=b\}
$$
denotes
$$
(discrete\ space\ on\ two\ points)\longrightarrow (Sierpinski\ space)\longrightarrow (antidiscrete\ space)\longrightarrow (single\ point)
$$
In $A \longrightarrow B$, each object and each morphism in $A$ necessarily appears in \ensuremath{B} as well; we avoid listing
the same object or morphism twice. Thus
both
$$
\{a\} \longrightarrow \{a,b\}\text{ and } \{a\} \longrightarrow \{b\}
$$ denote the same map from a single point to the discrete space with two points.
Both
$$\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}\text{ and }\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{U=x=V\}$$
denote the morphism gluing together points $U,x,V$.
In $\{a{\small\searrow}b\}$, the point $a$ is open and point \ensuremath{b} is closed.
Finally, for a class $C$ of morphisms in $Top$, let $$
% C_{0 \}^r$
%\item[] a group \ensuremath{F} is free iff $0\longrightarrow F$ is in $\{0\longrightarrow \ZZ\}^{rl}$
%\item[] a homomorphism $f$ is split iff $f \in \{\, 0\longrightarrow \ensuremath{G} : G\text{ arbitrary} \}^r$
%%+ a subgroup A is pure in a group $H$ iff $A\longrightarrow H$ is in $\{ n\ZZ\longrightarrow \ZZ : n>0 \}^r$
%\end{itemize}
%\item[ (v)] in the category of metric spaces and uniformly continuous maps,\\
% a metric space \ensuremath{X} is complete iff $\{1/n\}_n\longrightarrow \{1/n\}_n\cup \{0\} \,\rightthreetimes\, X\longrightarrow \{0\}$
% where the metric on $\{1/n\}_n$ and $\{1/n\}_n\cup \{0\}$ is induced from the
% real line\\
% a subset $A \subset X$ is closed iff $\{1/n\}_n\longrightarrow \{1/n\}_n\cup \{0\} \,\rightthreetimes\, A\longrightarrow X$
%\item[ (vi)] in the category of topological spaces,\\
% for a connected topological space X, each function on \ensuremath{X} is bounded
% iff %\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, |_|_n (-n,n) \longrightarrow R
% $$ \emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \cup_n (-n,n) \longrightarrow \RR$$
%%\newpage
\item[(i)]% (vii)]
In the category of topological spaces, the following holds: %(see notation defined below),
%\begin{itemize}
\item[] a Hausdorff space $K$ is compact iff $K\longrightarrow \{*\}$ is in $((\{a\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr}$
\item[] a Hausdorff space $K$ is compact iff $K\longrightarrow \{*\}$ is in $$
\{\, \{a\leftrightarrow b\}\longrightarrow \{a=b\},\, \{a{\small\searrow}b\}\longrightarrow \{a=b\},\,
\{b\}\longrightarrow \{a{\small\searrow}b\},\,\{a{\small\swarrow}o{\small\searrow}b\}\longrightarrow \{a=o=b\}\,\,\}^{lr}$$
\item[] a space $D$ is discrete iff $ \emptyset \longrightarrow D$ is in $ (\emptyset\longrightarrow \{*\})^{rl} $
\item[] a space $D$ is antidiscrete iff $ \ensuremath{D} \longrightarrow \{*\} $ is in %\begin{center}
$(\{a,b\}\longrightarrow \{a=b\})^{rr}= (\{a\leftrightarrow b\}\longrightarrow \{a=b\})^{lr} $ %\end{center}
\item[] a space $K$ is connected or empty iff $K\longrightarrow \{*\}$ is in $(\{a,b\}\longrightarrow \{a=b\})^l $
\item[] a space $K$ is totally disconnected and non-empty iff $K\longrightarrow \{*\}$ is in $(\{a,b\}\longrightarrow \{a=b\})^{lr} $
\item[] a space $K$ is connected and non-empty
iff
for some arrow $\{*\}\longrightarrow K$\\
$\text{ \ \ \ \ \ } \{*\}\longrightarrow K$ is in
$ (\emptyset\longrightarrow \{*\})^{rll} = (\{a\}\longrightarrow \{a,b\})^l$
\item[] a space $K$ is non-empty iff $K\longrightarrow \{*\}$ is in $ (\emptyset\longrightarrow \{*\})^l$
\item[] a space $K$ is empty iff $K \longrightarrow \{*\}$ is in $ (\emptyset\longrightarrow \{*\})^{ll}$
\item[] a space $K$ is $T_0$ iff $K \longrightarrow \{*\}$ is in $ (\{a\leftrightarrow b\}\longrightarrow \{a=b\})^r$
\item[] a space $K$ is $T_1$ iff $K \longrightarrow \{*\}$ is in $ (\{a{\small\searrow}b\}\longrightarrow \{a=b\})^r$
\item[] a space $X$ is Hausdorff iff for each injective map $\{x,y\} \hookrightarrow X$
it holds $\{x,y\} \hookrightarrow \ensuremath{X} \,\rightthreetimes\, \{ \ensuremath{x} {\small\searrow} \ensuremath{o} {\small\swarrow} \ensuremath{y} \} \longrightarrow \{ x=o=y \}$
\item[] a non-empty space $X$ is regular (T3) iff for each arrow $ \{x\} \longrightarrow X$ it holds
$ \{x\} \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{x{\small\searrow}X{\small\swarrow}U{\small\searrow}F\} \longrightarrow \{x=X=U{\small\searrow}F\}$
\item[] a space $X$ is normal (T4) iff $\emptyset \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}$
\item[] a space $X$ is completely normal iff $\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, [0,1]\longrightarrow \{0{\small\swarrow}x{\small\searrow}1\}$
where the map $[0,1]\longrightarrow \{0{\small\swarrow}x{\small\searrow}1\}$ sends $0$ to $0$, $1$ to $1$, and the rest $(0,1)$ to $x$
\item[] a space $X$ is path-connected iff $\{0,1\} \longrightarrow [0,1] \,\rightthreetimes\, \ensuremath{X} \longrightarrow \{*\}$
\item[] a space $X$ is path-connected iff for each Hausdorff compact space $K$ and each injective map $\{x,y\} \hookrightarrow K$ it holds
$\{x,y\} \hookrightarrow \ensuremath{K} \,\rightthreetimes\, \ensuremath{X} \longrightarrow \{*\}$
\item[] $(\emptyset\longrightarrow \{*\})^r$ is the class of surjections
\item[] $(\emptyset\longrightarrow \{*\})^{rr}$ is the class of subsets, i.e. injective maps $A\hookrightarrow B$ where the topology on $A$ is induced from $B$
\item[] $(\emptyset\longrightarrow \{*\})^{lrl}$ is the class of maps $A\longrightarrow B$ which split
\item[] $(\{b\}\longrightarrow \{a{\small\searrow}b\})^l$ is the class of maps with dense image
\item[] $(\{b\}\longrightarrow \{a{\small\searrow}b\})^{lr}$ is the class of closed subsets $A \subset X$, $A$ a closed subset of $X$
%+ $(\emptyset\longrightarrow \{*\})^l$ is the class of maps $A\longrightarrow B$ such that $A=B=\emptyset$ or $A\neq\emptyset$, $B$ arbitrary
%+ $(\emptyset\longrightarrow \{*\})^{ll}$ is the class of maps $A\longrightarrow B$ such that either $A=\emptyset$ or the map is an isomorphism
%+ $(\emptyset\longrightarrow \{*\})^r$ is the class of surjections
%+ $(\emptyset\longrightarrow \{a\})^{rl}$ is the class of maps of form $A\longrightarrow A\cup D$, $D$ is discrete
%%\item[] $((\{a\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr}$ is roughly the class of proper maps
%% (see Claim~2 below for exact statement).
%\end{itemize}
\item[ (ii)] in the category of topological spaces,\\
for a connected topological space X, each function on \ensuremath{X} is bounded
iff %\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, |_|_n (-n,n) \longrightarrow R
$$ \emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \cup_n (-n,n) \longrightarrow \RR$$
%\newpage
\item[ (iii)] in the category of metric spaces and uniformly continuous maps,\\
a metric space \ensuremath{X} is complete iff $\{1/n\}_n\longrightarrow \{1/n\}_n\cup \{0\} \,\rightthreetimes\, X\longrightarrow \{0\}$
where the metric on $\{1/n\}_n$ and $\{1/n\}_n\cup \{0\}$ is induced from the
real line\\
a subset $A \subset X$ is closed iff $\{1/n\}_n\longrightarrow \{1/n\}_n\cup \{0\} \,\rightthreetimes\, A\longrightarrow X$
\end{itemize}
\end{claim}
{\bf
Proof.
}
We defer the proof of the first two items to the next claim. The rest is straightforward to verify using the definitions.
\qed
%In (iv), we use that a finite group $H$ is nilpotent iff the diagonal $\{
%(h,h) : \ensuremath{h } \in \ensuremath{H} \}$ is subnormal in $H\times H$. (vii) is discussed below. QED.
%
%%Appendix~A shows that the usual formulations of the separation axioms are also Quillen %in fact
%%lifting properties.
%
Many of the separation axioms can be expressed as lifting properties with respect to maps
involving up to \ensuremath{4} points and the real line, see [Appendix A].
%\begin{problem}
%%Problem 0.
%Find more examples of meaningful lifting properties in various categories.
% Play with natural classes of morphisms to see whether their iterated orthogonals
% are meaningful.
%\end{problem}
%
%\begin{remark} Most of the definitions above are in a form useful in a diagram chasing computation.
%Let us comment on this.
%
%In category theory it is usual to view an object $X$ as either
%the arrow $\perp\longrightarrow X$ or $X\lra\top$ from the initial object $\perp$ or to the terminal object $\top$.
%However, in Claim~1(vii) we sometimes view (the properties of) a space as (properties of a non-unique!) arrow $\{*\} \longrightarrow X$ or
%$\{a,b\} \hookrightarrow X$. Our purpose is to give definitions useful in a diagram chasing computation,
%and these definitions can be used in this way.
%
%A group $G$ has cohomological dimension \ensuremath{1} iff each surjections $G'\longrightarrow G$ splits. Item (iv) views this
%as the following diagram chasing rule:
% in (the valid diagram corresponding to) $0\longrightarrow \ZZ \,\rightthreetimes\, G'\longrightarrow G$, it is permissible to replace $\ZZ$ by
% an arbitrary group $A$ (thereby obtaining a valid diagram corresponding to $0\longrightarrow A \,\rightthreetimes\, G'\longrightarrow G$).
%
%\end{remark}
%
%\begin{remark}\label{sTop:1}
%%Remark.
%Claim (v) shows that a computer-generated proof in (Ganesalingam, Gowers; Problem 2)
% of the claim that completeness is inherited by closed subsets of metric spaces, i.e.
% a closed subspace of a complete metric space is necessarily complete,
% translates to two applications of a diagram chasing rule
% corresponding to the lifting property.
%
%In fact, in any category for an arbitrary class $C$ of morphisms it holds
% $ X\longrightarrow \{*\} \,\in\, C^l$ and $f: A \hookrightarrow \ensuremath{X} \,\in\, C^l $ implies
%$A \longrightarrow \{*\} \,\in\, C^l $ whenever $f$ is a monomorphism and where $\{*\}$ denotes the terminal object.
%\end{remark}
%
%
%Note that, %unlike in some of the other examples,
%the definition involved infinite objects
%which are infinite sequences
%$\{1/n\}_n\longrightarrow \{1/n\}_n\cup \{0\} $. This can be probably be avoided if instead we
%consider {\em uniform spaces}, equivalently
%{\em type-definable equivalence relations}
%as {\em simplicial objects in the category
%of topological spaces} (see Remark~\ref{simp:obj:unifrom} and Conjecture~2 for details)
%and interpret the lifting property in %a subcategory of
%the category of simplicial objects
%in the category of topological spaces;
%this uses that a Cauchy filter is (almost) a map
%from a ``constant'' simplicial object.
%
%Conjecture~2 %This would give
%suggests a formal analogy corresponding to
%the informal analogy between {\em compact} topological spaces and
%{\em complete} metric (uniform) spaces.
%
%\begin{remark}
%%Remark.
%Claim demonstrates that a number of elementary notions can be concisely
% expressed in terms of a simple diagram chasing rule. However,
% it appears there is no (well-known) logic or proof system
% based on diagram chasing in a category. We make some suggestions towards
% such a proof system in the last two sections.
%\end{remark}
%
%\section{
%Elementary topological properties via finite topological spaces
%}
%
%First we must introduce notation for maps of finite topological spaces
%we use. Two features are important for us:
%\begin{itemize}
%\item[ 1.] it reminds one that a finite topological space is a category
% (degenerate if you like)
%\item[ 2.] it does not allow one to talk conveniently about non-identity
% endomorphisms of finite topological spaces. We hope this may help define
% a decidable fragment of elementary topology because there is
% a decidable fragment of diagram chasing without endomorphisms, see [GLZ].
%\end{itemize}
%
%A topological space comes with a {\em specialisation preorder} on its points: for
%points $x,y \in X$, $x \leq y$ iff $y \in cl x$ , or equivalently, a category whose
%objects are points of \ensuremath{X} and there is a unique morphism $x{\small\searrow}y$ iff $y \in cl x$.
%
%For a finite topological space X, the specialisation preorder or equivalently
%the category uniquely determines the space: a subset of \ensuremath{X} is closed iff it is
%downward closed, or equivalently, there are no morphisms going outside the
%subset.
%
%
%The monotone maps (i.e. functors) are the continuous maps for this topology.
%
%We denote a finite topological space by a list of the arrows (morphisms) in
%the corresponding category; '$\leftrightarrow $' denotes an isomorphism and '$=$' denotes
%the identity morphism. An arrow between two such lists denotes
%a continuous map (a functor) which sends each point to the correspondingly
%labelled point, but possibly turning some morphisms into identity morphisms,
%thus gluing some points.
%
%Thus, each point goes to "itself" and $$
% \{a,b\}\longrightarrow \{a{\small\searrow}b\}\longrightarrow \{a\leftrightarrow b\}\longrightarrow \{a=b\}
%$$
%denotes
%$$
% (discrete\ space\ on\ two\ points)\longrightarrow (Sierpinski\ space)\longrightarrow (antidiscrete\ space)\longrightarrow (single\ point)
%$$
%
%In $A \longrightarrow B$, each object and each morphism in $A$ necessarily appears in \ensuremath{B} as well; we avoid listing
%the same object or morphism twice. Thus
%both
%$$
%\{a\} \longrightarrow \{a,b\}\text{ and } \{a\} \longrightarrow \{b\}
%$$ denote the same map from a single point to the discrete space with two points.
%Both
% $$\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}\text{ and }\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{U=x=V\}$$
%denote the morphism gluing points $U,x,V$.
%
%In $\{a{\small\searrow}b\}$, the point $a$ is open and point \ensuremath{b} is closed.
%
%
%Let $$
%% C_{m>3$ such that the inclusion is strict.
An example using cofinite topology (suggested by Sergei Kryzhevich)
shows that $C_T^{lr}$
% \{\, \{a\leftrightarrow b\}\longrightarrow \{a=b\},\, \{a{\small\searrow}b\}\longrightarrow \{a=b\},\,
% \{b\}\longrightarrow \{a{\small\searrow}b\},\,\{a{\small\swarrow}o{\small\searrow}b\}\longrightarrow \{a=o=b\}\,\,\}^{lr}
does not define the class of compact spaces: indeed, consider infinite sets $A\subset B$, $\omega\leq \card A<\card B$, equipped
with cofinite topology (i.e. a subset is closed iff it is finite).
Then $A\subseteq \ensuremath{B} \in C_T^l$ yet $A \subseteq \ensuremath{B} \,\rightthreetimes\, A\longrightarrow \{*\}$ fails:
for a map $f:B\longrightarrow A$ the preimage of some (necessarily closed) point is infinite as $\card \ensuremath{B} > \card A$, hence not closed,
and the map is not continious. Hence, $A\longrightarrow \{*\} \,\notin\, C_T$ yet $A$ is compact (non-Hausdorff).
This example could probably be generalised to show that
that $((\{a\}\longrightarrow \{a{\small\searrow}b\})^r_{<4})^{lr} \subsetneq ((\{a\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr} $.
\end{remark}
\begin{problem}
%Problem.
\begin{itemize}
\item[(a)]Calculate $$\!\!\!\!\!\!\!\!\!\!\!\! ((\{b\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr},\ ((\{b\}\longrightarrow \{a{\small\searrow}b\})^{lrr},\ \text{and}\ $$ $$
(\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\})^{lr} $$
Could either be viewed as a ``definition" of the real line?
\item[(b)]Characterise the interval $[0,1]$, a circle $\mathbb S^1$ and, more generally, spheres $\mathbb S^n$ using their topological characterisations
provided by the Kline sphere charterisation theorem and its analogues. An example of such a characterisation is
that a topological space $X$ is homomorphic to the circle $\mathbb S^1$ iff
$X$ is a connected Hausdorff metrizable space such that $X\setminus \{x,y\}$ is not connected for any two points $x\neq y\in X$
([Hocking,Young, Topology, Thm.2-28,p.55]); another example is that a topological space $X$ is homomorphic to
the closed interval $[0,1]$ iff
$X$ is a connected Hausdorff metrizable space such that $X\setminus \{x\}$ is not connected for exactly two points $x\neq y\in X$
([Hocking,Young, Topology, Thm.2-27,p.54]).
\end{itemize}
\end{problem}
%\begin{remark}
%%Remark.
% Urysohn lemma and Tietze extension theorem relate lifting properties involving $\RR$
% and those involving opens maps of finite topological spaces,
% and this is why we hope the question above might have something to
% do with the real line. Let us give some more details.
%
% Note a map \ensuremath{f} of finite spaces is open iff \ensuremath{f} is in $(\{b\}\longrightarrow \{a{\small\searrow}b\})^r$,
% and that $\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}$ is an open map.
%
% A space \ensuremath{X} is normal (T4) iff $\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}$, hence
% the Uryhson lemma can be stated as follows:
% $$
%\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}$$ iff
%$$\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{0'\} \cup [0,1] \cup \{1'\} \longrightarrow \{0=0'{\small\searrow}x{\small\swarrow}1=1'\}$$
% where points $0',0$ and $1,1'$ are topologically indistinguishable in
% $\{0'\} \cup [0,1] \cup \{1'\}$,
% the closed interval $[0,1]$ goes to $x$, and $0,0'$ map to point $0=0'$, and $1,1'$
% map to point $1=1'$.
%
% Tietze extension theorem states that for a normal space \ensuremath{X} and a closed subset A of X,
% $A\longrightarrow \ensuremath{X} \,\rightthreetimes\, \RR\longrightarrow \{*\} $, i.e. in notation
% $\RR\longrightarrow \{*\}$ is in $$\left( (\{b\}\longrightarrow \{a{\small\searrow}b\})^{lr}\cap \{ A\longrightarrow \ensuremath{X} : \emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\} \right)^r$$
%
% Note that $\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, [0,1]\longrightarrow \{0{\small\searrow}x{\small\swarrow}1\}$ is the stronger property \ensuremath{X} is
% perfectly normal. A normal space \ensuremath{X} is perfectly normal provided
% each closed subset of \ensuremath{X} is the intersection of a countably many open subsets.
%
% For example, is $\{0'\} \cup [0,1] \cup \{1'\} \longrightarrow \{0=0'{\small\searrow}x{\small\swarrow}1=1'\}$ in
% $(\{a{\small\swarrow}U{\small\searrow}x{\small\swarrow}V{\small\searrow}b\}\longrightarrow \{a{\small\swarrow}U=x=V{\small\searrow}b\}^{lr} \subset
% ((\{b\}\longrightarrow \{a{\small\searrow}b\})^r_{<5})^{lr} $?
%\end{remark}
%
\begin{remark}
%Problem.
Is there a model category or a factorisation system of interest
associated with any of these lifting properties, for example compactness/properness?
\end{remark}
Many of the separation axioms can be expressed as lifting properties with respect to maps
involving up to \ensuremath{4} points and the real line, see [Appendix A].
\subsection{Hausdorff axioms of topology as diagram chasing computations with finite categories
}
Above we reformulated a number of elementary notions in topology in terms of preorders.
Now we observe that the original axioms of topology as formulated by Hausdorff
can also be viewed as rules for manipulating
finite preorders.
Early works talk of topology in terms of {\em neighbourhood} systems $U_x$
where $U_x$ varies though
{\em open neighbourhoods of points} of a topological space; this is how
the notion of topology was defined by Hausdorff. In the notation of arrows,
{\em a neighbourhood system $U_x$, $x\in X$} would correspond to a system of arrows
$$
\{x\}\longrightarrow X\xrightarrow{\,\,\, U\,\,\,}\{x{\small\searrow}x'\}$$
and Hausdorff's axioms (A),(B),(C) (see Appendix~B) would correspond to
diagram chasing rules.
Here we show the axioms of topology stated in the more modern
language of open subsets %The Hausdorff axioms of a topological space
can be seen as diagram chasing rules for manipulating diagrams
involving notation such as
$$ \{x\}\longrightarrow X,\, X\longrightarrow \{x{\small\searrow}y\},\, X\longrightarrow \{x\leftrightarrow y\} $$
in the following straightforward way; cf. [Gavrilovich, Elementary Topology,\S.2.1] for more
details.
As is standard in category theory, identify a point $x$ of a topological space $X$
with the arrow $\{x\}\longrightarrow X$, a subset $Z$ of $X$ with the arrow $X\longrightarrow \{z\leftrightarrow z'\}$,
and an open subset $U$ of $X$ with the arrow $X\longrightarrow \{u{\small\searrow}u'\}$.
With these identifications, the Hausdorff axioms of a topological space become
rules for manipulating such arrows, as follows.
{\em Both the empty set and the whole of \ensuremath{X} are open} says that the compositions
$$ X\longrightarrow \{c\}\longrightarrow \{o{\small\searrow}c\}\text{ and }X\longrightarrow \{o\}\longrightarrow \{o{\small\searrow}c\} $$
behave as expected (the preimage of \{o\} is empty under the first map,
and is the whole of \ensuremath{X} under the second map).
{\em The intersection of two open subsets is open} means the arrow
$$ X\longrightarrow \{o{\small\searrow}c\}\times\{o'{\small\searrow}c'\} $$
behaves as expected (the ``two open subsets'' are the preimages of points $o\in\{o{\small\searrow}c\}$ and $o'\in\{o'{\small\searrow}c'\}$;
``the intersection'' is the preimage of $(o,o')$ in
$\{o{\small\searrow}c\}\times\{o'{\small\searrow}c'\}$ ).
Finally, {\em a subset $U$ of $X$ is open iff each point $u$ of $U$ has an open
neighbourhood inside of $U$}
corresponds to the following diagram chasing rule:
%+for each arrow h:X\longrightarrow \{U\leftrightarrow U'\},
%+
%+ \{U{\small\searrow}U'\}
%+ \cap |
%+(exist) / |
%+ / |
%+ / |
%+ / v
%+ \ensuremath{X} --h\longrightarrow \{U\leftrightarrow U'\}
%+
%+iff
%+
%+
%+\{u\}---\longrightarrow \{u{\small\searrow}U\leftrightarrow U'\}
%+ | \cap |
%+ |(exist)/ |
%+ | / |
%+ | / |
%+ \ensuremath{v} / v
%+ \ensuremath{X} --h\longrightarrow \{u=U\leftrightarrow U'\}
%+
%+here the arrows $\{U\}\longrightarrow \{U{\small\searrow}U'\}$
%+and $\{U{\small\searrow}U'\}\longrightarrow \{U\leftrightarrow U'\}$ denote the obvious maps.
%\input diagram_open_subsets_3arrow.tex
for each arrow $X\xra[\xi_U]{} \{U\longleftrightarrow \bar U \}$ it holds\\
$ \xymatrix{ { } & \{U\ra\bar U\} \ar[d] \\
{X} \ar[r]_{\!\!\!\!\!\!\!\!\!\xi_U} \ar@{-->}[ur] & { \{U\longleftrightarrow \bar U \} }}$
\ \ \ \ iff for each $\{u\}\lra X$, \ \ \ \ \ \ \
$ \xymatrix{ {\{u\}} \ar[r] \ar[d] & \{u \ra U \longleftrightarrow \bar U\} \ar[d] \\
{X} \ar[r]_{\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\xi_U} \ar@{-->}[ur] & { \{u=U\longleftrightarrow \bar U \} }}$
%
%\begin{itemize}
%\item[]
%
%\item[($3_\rightarrow$)] %a set $U$ is open iff for every point $u\in U$ there is an open subset $U'$ such that $u\in U'\subset U$.
%for each arrow $X\xra[\xi_U]\\{\\} \{U\llrra \bar \ensuremath{U} \}$ it holds\\
% $ \xymatrix{ \{ \} & \{U\ra\bar U\} \ar[d] \\
%\{X\} \ar[r]_{\!\!\!\!\!\!\!\!\!\xi_U} \ar@{\longrightarrow }[ur] & { \{U\llrra \bar \ensuremath{U} \} }}$
%\ \ \ \ iff for each $\{u\}\lra X$, \ \ \ \ \ \ \
% $ \xymatrix{ {\{u\}} \ar[r] \ar[d] & \{u \ra \ensuremath{U} \llrra \bar U\} \ar[d] \\
%\{X\} \ar[r]_{\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\xi_U} \ar@{\longrightarrow }[ur] & { \{u=U\llrra \bar \ensuremath{U} \} }}$
%
%\item
%
%a set $U$ is open iff for every point $u\in U$ there
%is an open subset $U'$ such that $u\in U'\subset U$.
%for each arrow $X\xra[\xi_U]\\{\\} \{U\longleftrightarrow \bar \ensuremath{U} \}$ it holds\\
% $ \xymatrix{ \{ \} & \{U\ra\bar U\} \ar[d] \\
%\{X\} \ar[r]_{\!\!\!\!\!\!\!\!\!\xi_U} \ar@{ \longrightarrow }[ur] & { \{U\longleftrightarrow \bar \ensuremath{U} \} }}$
%\ \ \ \ iff for each $\{u\}\lra X$, \ \ \ \ \ \ \
% $ \xymatrix{ {\{u\}} \ar[r] \ar[d] & \{u \ra \ensuremath{U} \longleftrightarrow \bar U\} \ar[d] \\
%\{X\} \ar[r]_{\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\xi_U} \ar@{ \longrightarrow }[ur] & {
%\{u=U\longleftrightarrow \bar \ensuremath{U} \} }}$
%\end{itemize}
%
{\em The preimage of an open set is open} corresponds to the composition $$
X\longrightarrow Y\longrightarrow \{u{\small\searrow}u'\}\longrightarrow \{u\leftrightarrow u'\} . $$
This observation suggests that some arguments in elementary topology may be
understood entirely in terms of diagram chasing, see [Gavrilovich, Elementary Topology] for some
examples. We hope that this reinterpretation may help clarify the nature of the axioms of a
topological space, in particular it offers a constructive approach, may clarify
to what extent set-theoretic language is necessary, and perhaps help to suggest
an approach to ''tame topology'' of Grothendieck,
%, Does this lead to tame topology of Grothendieck,
i.e.~a foundation of topology ``without false
problems" and "wild phenomena" "at the very beginning".
%Let us state two problems; we hope they help to clarify the notion
%of an ergosystem and that of a topological space.
%
%
%
%\begin{problem}%[Problem 1.]
%\label{problem:1}
%Write a short program which extracts diagram chasing derivations
%from texts on elementary topology, in the spirit of the ideology of
%ergosystems/ergostructures.
%\end{problem}
%
%\begin{problem}\label{problem:2}
%Develop elementary topology in terms of finite categories (viewed
%as finite topological spaces) and labelled commutative diagrams, with an
%emphasis on labels (properties) of morphisms defined by the Quillen lifting
%property. Does this lead to tame topology of Grothendieck, i.e. a foundation
%of topology "without false problems" and "wild phenomena" "at the very
%beginning" ?
%\end{problem}
%
%\begin{problem}
%%_Problem 3_.
%(Ganesalingam, Gowers) wrote an automatic theorem prover trying to
%make it "thinking in a human way". In a couple of their examples their
%generated proofs amount to diagram chasing, e.g. Claim (v) shows the generated
%proof of the claim that a closed subspace of a complete metric space is
%necessarily complete translates to two applications of a diagram chasing rule
%corresponding to the lifting property, see [Gavrilovich, Slides] for details.
%Arguably, both examples on the first page of (ibid.) also correspond to diagram
%chasing. In their approach, can this be seen as evidence that humans are
%really thinking by diagram chasing and the lifting property in particular? Note
%that Claim (v) involves examples typically shown to students to clarify the
%concepts of a metric space being complete or closed. Can one base a similar
%theorem prover on our observations, particularly in elementary topology?
%\end{problem}
%
%
%Let us comment on an approach to Problem 1.
%
%
%We observed that there is a simple rule which leads to several notions in
%topology interesting enough to be introduced in an elementary course. Can this
%rule be extended to a very short program which learns elementary topology?
%
%We suggest the following naive approach is worth thinking about.
%
%
%
%The program maintains a collection of directed labelled graphs and certain
%distinguished subgraphs. Directed graphs represent parts of a category;
%distinguished subgraphs represent commutative diagrams. Labels represent
%properties of morphisms. Further, the program maintains a collection of rules
%to manipulate these data, e.g. to add or remove arrows and labels.
%
%The program interacts with a flow of signals, say the text
%of [Bourbaki, General Topology, Ch.1], and seeks correlations
%between the diagram chasing rules and the flow of signals.
%It finds a "correlation" iff certain strings occur nearby
%in the signal flow iff they occur nearby in a diagram chasing rule.
%To find "what's interesting", by brute force it searches
%for a valid derivation which exhibits such correlations. To guide the search
%and exhibit missing correlations in a derivation under consideration, it may
%ask questions: are these two strings related? Once it finds such a derivation,
%the program "uses it for building its own structure". Labels correspond to
%properties of morphisms. Labels defined by the lifting property play an
%important role, often used to exclude counterexamples making a diagram chasing
%argument fail. In [DeMorgan] we analysed the text of the definitions of
%surjective and injective maps showing what such a correlation may look like in
%a "baby" case.
%
%A related but easier task is to write a theorem prover doing diagram chasing
%in a model category. The axioms of a (closed or not) model category
%as stated in [Quillen,I.1.1] can be interpreted as rules to manipulate
%labelled commutative diagrams in a labelled category. It appears
%straightforward how to formulate a logic (proof system) based on these rules
%which would allow to express statements like: Given a labelled commutative diagram,
%(it is permissible to) add this or that arrow or label.
%Moreover, it appears not hard to write a theorem prover for this logic
%doing brute force guided search. What is not clear whether this logic is complete
%in any sense or whether there are non-trivial inferences of this form to prove.
%
%Writing such a theorem prover is particularly easy when the underlying category
%of the model category is a partial order [GavrilovichHasson] and [BaysQuilder] wrote
%some code for doing diagram chasing in such a category. However, the latter
%problem is particularly severe as well.
%
%
%The two problems are related; we hope they help to clarify the notion
%of an ergosystem and that of a topological space.
%
%
%
%The following is a more concrete question towards Problem~\ref{problem:2}.
%
%\begin{problem}
% (a) Prove that a compact Hausdorff space is normal by diagram chasing;
% does it require additional axioms?
% Note that we know how to express the statement entirely in terms of the
% lifting property and finite topological spaces of small size.
%
% (b) Formalise the argument in [Fox, 1945] which implies the category of
% topological spaces is not Cartesian closed. Namely, Theorem \ensuremath{3} [ibid.] proves that
% if \ensuremath{X} is separable metrizable space, \ensuremath{R} is the real line, then
% \ensuremath{X} is locally compact
% iff
% there is a topology on $X^\RR$ such that for any space $T$,
% a function $h:X\times T\longrightarrow \RR$ is continuous iff
% the corresponding function $h^*:T\longrightarrow X^\RR$ is continuous
% (where $h(x,t)=h^*(t)(x))$
% Note that here we do not know how to express the statement.
%\end{problem}
%
%\begin{remark}\label{simp:obj:unifrom}
%%Remark.
% In [Gavrilovich, Tame Topology,\S6] and [Gavrilovich, Elementary Topology,\S2.9] we note that
% uniform spaces may be viewed as simplicial objects in
% the category of topological spaces in the following way.
% We wish to emphasise that this observation can be easily "read off"
% from [Bourbaki, General Topology, II\S1.1.1] if one is inclined
% to translate everything into diagram chasing.
%
% Let \ensuremath{X} be a set. Let
% %X\leq >\ensuremath{X\times X}\Longleftrightarrow \ensuremath{X\times X\times X}\Longleftrightarrow ....
% $$X\Longleftrightarrow X\times \ensuremath{X} \Longleftrightarrow X\times X\times \ensuremath{X} ...$$
%be the "trivial" simplicial set where degeneracy and face maps are
% coordinate projections and diagonal embeddings.
% A uniform structure on a set $X$ is a filter, hence a topology, on the set \ensuremath{X\times X}
% satisfying certain properties; equip \ensuremath{X\times X} with this topology.
% Put the antidiscrete topology on X; put on \ensuremath{X\times X\times X} the topology which is
% the pullback of the topologies on \ensuremath{X\times X} and \ensuremath{X} along the {\em two} projection maps
% on two of the three pairs of coordinates, and
% similarly for \ensuremath{X\times X\times X\times X} etc. A straightforward verification shows this is well-defined
% whenever the topology on \ensuremath{X\times X} corresponds to a uniform structure on $X$.
%
%We\footnote{I thank Martin Bays for this observation} note that the uniform structure can be viewed model-theoretically
% as being the same thing as a
%a binary relation defined as the conjunction of infinitely many formulas such that in a saturated model it defines an equivalence relation.
%\end{remark}
%
%\begin{remark}
%%Remark.
%In [Gavrilovich, Tame Topology, \S5.4] we observe that a number of consequences
% of compactness can be expressed as a change of order of quantifiers in a
% formula,
% i.e. are of form %+AE \phi(..) \Longrightarrow EA \phi (..) ,
% $\forall \exists \phi(...)\implies\exists\forall \phi(...)$
% namely that a real-valued function on a compact is necessarily bounded,
% that a Hausdorff compact is necessarily normal,
% that the image in \ensuremath{X} of a closed subset in $X\times K$ is necessarily closed,
% Lebesgue's number Lemma, and paracompactness.
%
% Such formulae correspond to inference rules of a special form,
% and we feel a special syntax should be introduced to state
% these rules.
%
%
% For example, consider the statement that
% "a real-valued function on a compact domain is necessarily bounded".
% As a first order formula, it is expressed as
%%+
%%+ for all \ensuremath{x} in \ensuremath{K} there exists M ( f(x) \leq M ) \Longrightarrow there exists M for all \ensuremath{x} in \ensuremath{K} ( f(x) \leq M )
%$$
% \forall \ensuremath{x} \in \ensuremath{K} \exists M ( f(x) \leq M ) \Longrightarrow \exists M \forall \ensuremath{x} \in \ensuremath{K} ( f(x) \leq M )
%$$
% Another way to express it is:
%%+
%%+ there exists M:K\longrightarrow \RR for all \ensuremath{x} in \ensuremath{K} ( f(x) \leq M(x) ) \Longrightarrow there exists M in \RR for all \ensuremath{x} in \ensuremath{K} ( f(x) \leq M )
%$$
% \exists M:K\longrightarrow \RR \forall \ensuremath{x} \in \ensuremath{K} ( f(x) \leq M(x) ) \Longrightarrow \exists M \in \RR \forall \ensuremath{x} \in \ensuremath{K} ( f(x) \leq M )
%$$
% Note that all that happened here is that a function $M:K\longrightarrow \RR$,
% become a constant $M \in \RR$, or rather
% expression "M(x)" of type $K\longrightarrow M$ which used (depended upon) variable "x"
% become expression "M" which does not use (depend upon) variable "x".
%
% We feel there should be a special syntax which would allow
% to express above as an inference rule {\em removing dependency of "M(x)" on "x"},
% and this syntax should be used to express consequences of compactness
% in a diagram chasing derivation system for elementary topology.
%
% To summarise, we think that compactness should be formulated
% with help of inference rules for expressly manipulating which variables are 'new',
% in what order they 'were' introduced,
% and what variables terms depend on, e.g. rules replacing a term t(x,y) by term t(x).
%
% Something like the following:
%
%\begin{verbatim}
% ... f(x) =< M(x) ...
% --------------------
% ... f(x) =< M ...
%\end{verbatim}
%\end{remark}
%
%\section{
% Ergo-Structures/Ergo-Systems Conjecture of Misha Gromov.
%}
%
%We conclude with a section which aims to explain our motivation and
%hence is speculative and perhaps somewhat inappropriate in what is mostly
%a mathematical text.
%
%
%Misha Gromov [Memorandum Ergo] conjectures
%there are particular mathematical, essentially combinatorial, structures,
%called {\em ergostructures} or {\em ergosystems},
%which help to understand complex biological behaviour including learning
%and create mathematically interesting models of these processes.
%
%We hope our observations may eventually help to uncover
%an essentially combinatorial reasoning behind elementary topology,
%and thereby suggest an example of an ergostructure.
%
%
%
%An ergosystem/ergostructure may be thought of as an "engine" producing
%(structurally or mathematically) {\em interesting} behaviour
%which is then misappropriated into a {\em useful} behaviour by a biological
%system.
%
%
%"Behaviour" is thought of as an interaction with a flow of signals.
%By itself, such an "engine" produces {\em interesting} behaviour,
%with little or no concern for any later use;
%it "interacts with an incoming flow of signals;
%it recognizes and selects what is {\em interesting} for itself
%in such a flow and uses it for building its own structure"
%[Gromov, Memorandum Ergo].
%
%
%
%An analogy might help. Consider a complex mechanical contraption powered by an
%engine, such as a loom. By itself, there is nothing directly useful done by its
%engine; indeed, the very same engine may be used by different mechanisms for
%all sorts of useful and useless tasks. To understand the workings of a
%mechanism, sometimes you had better forget its purpose and ask what is the
%engine, how is it powered, and what keeps the engine in good condition.
%Understanding the loom's engine (only) in terms of how it helps to weave is
%misleading.
%
%Thus, the concept of an ergostructure/ergosystems suggests a different kind of
%questions we should ask about biological systems and learning.
%
%
%A further suggestion is that these "engines" might be rather universal, i.e.
%able to behave interestingly interacting with a diverse range of signal flows.
%At the very beginning an ergosystem/structure is a "crisp" mathematically
%interesting structure of size small enough to be stumbled upon by evolution; as
%it grows, it becomes "fuzzy" and specialised to a particular kind of flow of
%signals.
%
%
%However, we want to draw attention to the following specific suggestion:
%\begin{quote}
% ""The category/functor modulated structures can not be directly used by ergosystems,
% e.g. because the morphisms sets between even moderate objects are
% usually unlistable.
% But the ideas of the category theory show that there are certain (often
% non-obviuos) rules for generating proper concepts. (You ergobrain would not
% function if it had followed the motto: "in my theory I use whichever definitions
% I like".) The category theory provides a (rough at this stage) hint on a possible
% nature of such rules.
% [Gromov, Ergobrain]
%\end{quote}
%Our observations give an example of a simple rule which can be used "to
%generate proper concepts", particularly in elementary topology. We hope that
%our observations can make the hint less rough, particularly if one properly
%develops elementary topology in terms of diagram chasing, with an emphasis on
%the lifting property.
%
%
%
%{\bf Problem 1.} Write a short program which extracts diagram chasing derivations
%from texts on elementary topology, in the spirit of the ideology of
%ergosystems/ergostructures. That is, it considers a flow of signals
%interesting if it correlates with diagram chasing rules.
%
%In the previous section we give some suggestions, albeit naive, what such a program
%might look like.
%
%{\bf Problem 2.} Develop elementary topology in terms of labelled commutative
%diagrams involving finite categories (viewed as finite topological spaces),
%with an emphasis on labels (properties) of morphisms defined by the Quillen
%lifting property.
%
% Does this lead to the tame topology of Grothendieck, i.e. a foundation of topology
%"without false problems" and "wild phenomena" "at the very beginning" ?
%
%
%In the previous section we give some suggestions, albeit naive, what such
%a program might look like and how to express elementary topology
%in terms of labelled diagram chasing.
%
Acknowledgements and historical remarks. To be written.
This work is a continuation of [DMG]; early history is given there. I thank
M.Bays, D.Krachun, K.Pimenov, V.Sosnilo, S.Synchuk and P.Zusmanovich for discussions and proofreading; I
thank L.Beklemishev, N.Durov, S.V.Ivanov, S.Podkorytov, A.L.Smirnov for discussions. I also
thank several students for encouraging and helpful discussions. Chebyshev
laboratory, St.Petersburg State University, provided a coffee machine and an
excellent company around it to chat about mathematics. Special thanks are to
Martin Bays for many corrections and helpful discussions. Several observations
in this paper are due to Martin Bays. I thank S.V.Ivanov for several
encouraging and useful discussions; in particular, he suggested to look at the
Lebesque's number lemma and the Arzela-Ascoli theorem. A discussion with Sergei
Kryzhevich motivated the group theory examples. The introduction has been
influenced by a discussion with with Dan Grayson.
Much of this paper was done in St.Petersburg; it wouldn't have been possible
without support of family and friends who created an excellent social
environment and who occasionally accepted an invitation for a walk or a coffee
or extended an invitation; alas, I made such a poor use of it all.
This note is elementary, and it was embarrassing and boring, and
embarrassingly boring, to think or talk about matters so trivial, but luckily
I had no obligations for a time.
\newpage
\section{
Appendix A. Separation axioms as lifting properties (from Wikipedia)
}
The separation axioms are lifting properties with respect to maps involving up to \ensuremath{4} points
and the real line. What follows below is the text of the Wikipedia page on the separation axioms
where we added lifting properties formulae expressing what is said there in words.
%[G] shows that a number of standard elementary notions
%of abstract topology can be defined by applying the lifting property to simple
%morphisms of finite topological spaces, and includes some speculations on the
%wider significance of this.
%
Let \ensuremath{X} be a topological space. Then two points \ensuremath{x} and \ensuremath{y} in \ensuremath{X} are {\em topologically distinguishable}
iff the map $\{x\leftrightarrow y\} \longrightarrow X$ is not continuous, i.e.
iff %if they do not have exactly the same neighbourhoods (or equivalently the same open neighbourhoods);
at least one of them has an open neighbourhood which is not a neighbourhood of the other.
Two points \ensuremath{x} and \ensuremath{y} are {\em separated} iff neither $ \{x{\small\searrow}y\} \longrightarrow X$ nor $\{x{\small\searrow}y\} \longrightarrow X$ is continuous,
i.e~each of them has a neighbourhood that is not a neighbourhood of the other;
in other words, neither belongs to the other's closure, $x \notin cl\, x$ and $y \notin cl\, x$.
More generally, two subsets A and \ensuremath{B} of \ensuremath{X} are {\em separated} iff each is disjoint from the other's closure,
i.e.~$ A\cap cl \ensuremath{B} = B\cap cl A = \emptyset $.
(The closures themselves do not have to be disjoint.) In other words, the map
$ i_{AB} : \ensuremath{X} \longrightarrow \{A \leftrightarrow \ensuremath{x} \leftrightarrow B\}$
sending the subset $A$ to the point $A$, the subset $B$ to the point $B$, and the rest to the point $x$,
factors both as
$$
X \longrightarrow \{A \leftrightarrow U_A {\small\searrow} \ensuremath{x} \leftrightarrow B\} \longrightarrow \{A=U_A \leftrightarrow \ensuremath{x} \leftrightarrow B\} $$
and
$$
X \longrightarrow \{A \leftrightarrow \ensuremath{x} {\small\swarrow} U_B \leftrightarrow B\} \longrightarrow \{A \leftrightarrow \ensuremath{x} \leftrightarrow U_B=B\} $$
here the preimage of $x,B$, resp. $x,A$ is a closed subset containing $B$, resp. $A$, and disjoint from $A$, resp. $B$.
All of the remaining conditions for separation of sets may also be applied to points (or to a point and a set)
by using singleton sets. Points \ensuremath{x} and \ensuremath{y} will be considered separated, by neighbourhoods,
by closed neighbourhoods, by a continuous function, precisely by a function,
iff their singleton sets $\{x\}$ and $\{y\}$ are separated according to the corresponding criterion.
Subsets A and \ensuremath{B} are {\em separated by neighbourhoods} iff
A and \ensuremath{B} have disjoint neighbourhoods, i.e.
iff
$ i_{AB} : \ensuremath{X} \longrightarrow \{A \leftrightarrow \ensuremath{x} \leftrightarrow B\}$ factors as $$
X \longrightarrow \{A \leftrightarrow U_A {\small\searrow} \ensuremath{x} {\small\swarrow} U_B \leftrightarrow B\} \longrightarrow \{A=U_A \leftrightarrow \ensuremath{x} \leftrightarrow U_B=B\} $$
here the disjoint neighbourhoods of A and \ensuremath{B} are the preimages of open subsets ${A,U_A}$ and ${U_B,B}$ of
$ \{A \leftrightarrow U_A {\small\searrow} \ensuremath{x} {\small\swarrow} U_B \leftrightarrow B\}$, resp.
They are {\em separated by closed neighbourhoods}
iff they have disjoint closed neighbourhoods, i.e.
$i_{AB}$ factors as
$$
X \longrightarrow \{A \leftrightarrow U_A {\small\searrow} U'_A {\small\swarrow} \ensuremath{x} {\small\searrow} U'_B {\small\swarrow} U_B \leftrightarrow B\} \longrightarrow \{A\leftrightarrow U_A=U'_A= \ensuremath{x} = U'_B=U_B\leftrightarrow B\}
.$$
They are {\em separated by a continuous function} iff
there exists a continuous function \ensuremath{f} from the space \ensuremath{X} to the real line $\RR$ such that $f(A)=0$ and $f(B)=1$,
i.e.
the map $i_{AB}$ factors as
$$
X \longrightarrow \{0'\} \union [0,1] \union \{1'\} \longrightarrow \{A \leftrightarrow \ensuremath{x} \leftrightarrow B\}
$$
where points $0',0$ and $1,1'$ are topologically indistinguishable,
and $0'$ maps to $A$, and $1'$ maps to $B$, and $[0,1]$ maps to $x$.
Finally, they are {\em precisely separated by a continuous function}
iff there exists a continuous function \ensuremath{f} from \ensuremath{X} to $\RR$ such that the preimage $f^{ - 1}(\{0\})= A$ and $f^{ - 1}(\{1\})=B$.
i.e.~iff $i_{AB}$ factors as $$
X \longrightarrow [0,1] \longrightarrow \{A \leftrightarrow \ensuremath{x} \leftrightarrow B\}
$$ where $0$ goes to point $A$ and $1$ goes to point $B$.
These conditions are given in order of increasing strength:
Any two topologically distinguishable points must be distinct, and any two separated points must be topologically distinguishable. Any two separated sets must be disjoint, any two sets separated by neighbourhoods must be separated, and so on.
%For more on these conditions (including their use outside the separation axioms), see the articles Separated sets and Topological distinguishability.
%Main definitions
The definitions below all use essentially the preliminary definitions above.
In all of the following definitions, \ensuremath{X} is again a topological space.
\begin{itemize}
\item[] \ensuremath{X} is T0, or Kolmogorov, if any two distinct points in \ensuremath{X} are topologically
distinguishable. (It will be a common theme among the separation axioms to have
one version of an axiom that requires T0 and one version that doesn't.)
As a formula, this is expressed as
$$ \{x\leftrightarrow y\} \longrightarrow \{x=y\} \,\rightthreetimes\, \ensuremath{X} \longrightarrow \{*\}$$
\item[] \ensuremath{X} is R0, or symmetric, if any two topologically distinguishable points in X
are separated, i.e.
$$\{x{\small\searrow}y\} \longrightarrow \{x\leftrightarrow y\} \,\rightthreetimes\, \ensuremath{X} \longrightarrow \{*\}$$
\item[] \ensuremath{X} is T1, or accessible or Frechet, if any two distinct points in \ensuremath{X} are
separated, i.e.
$$ \{x{\small\searrow}y\} \longrightarrow \{x=y\} \,\rightthreetimes\, \ensuremath{X} \longrightarrow \{*\} $$
Thus, \ensuremath{X} is T1 if and only if it is both T0 and R0. (Although you may
say such things as "T1 space", "Frechet topology", and "Suppose that the
topological space \ensuremath{X} is Frechet", avoid saying "Frechet space" in this
context, since there is another entirely different notion of Frechet space in
functional analysis.)
\item[] \ensuremath{X} is R1, or preregular, if any two topologically distinguishable points in
X are separated by neighbourhoods. Every R1 space is also R0.
\item[] \ensuremath{X} is weak Hausdorff, if the image of every continuous map from a compact
Hausdorff space into \ensuremath{X} is closed. All weak Hausdorff spaces are T1, and all
Hausdorff spaces are weak Hausdorff.
\item[] \ensuremath{X} is Hausdorff, or T2 or separated, if any two distinct points in \ensuremath{X} are
separated by neighbourhoods, i.e.
$$ \{x,y\} \hookrightarrow \ensuremath{X} \,\rightthreetimes\, \{x{\small\searrow}X{\small\swarrow}y\} \longrightarrow \{x=X=y\} $$
Thus, \ensuremath{X} is Hausdorff if and only if it is both T0
and R1. Every Hausdorff space is also T1.
\item[] \ensuremath{X} is $T2\frac12$, or Urysohn, if any two distinct points in \ensuremath{X} are separated by
closed neighbourhoods, i.e.
$$
\{x,y\} \hookrightarrow \ensuremath{X} \,\rightthreetimes\, \{x{\small\searrow}x'{\small\swarrow}X{\small\searrow}y'{\small\swarrow}y\} \longrightarrow \{x=x'=X=y'=y\}
$$
Every T2$\frac12$ space is also Hausdorff.
\item[] \ensuremath{X} is completely Hausdorff, or completely T2, if any two distinct points in
X are separated by a continuous function, i.e.
$$ \{x,y\} \hookrightarrow \ensuremath{X} \,\rightthreetimes\, [0,1]\longrightarrow \{*\}
$$ where $\{x,y\} \hookrightarrow X$ runs through all injective maps from the discrete two
point space $\{x,y\}$.
Every completely Hausdorff space is
also T2$\frac 12 ½$.
\item[] \ensuremath{X} is regular if, given any point \ensuremath{x} and closed subset $F$ in \ensuremath{X} such that \ensuremath{x} does
not belong to $F$, they are separated by neighbourhoods, i.e.
$$ \{x\} \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{x{\small\searrow}X{\small\swarrow}U{\small\searrow}F\} \longrightarrow \{x=X=U{\small\searrow}F\}
$$
(In fact, in a regular
space, any such \ensuremath{x} and\ensuremath{F} will also be separated by closed neighbourhoods.) Every
regular space is also R1.
\item[] \ensuremath{X} is regular Hausdorff, or T3, if it is both T0 and regular.[1] Every
regular Hausdorff space is also $T2\frac12$.
\item[] \ensuremath{X} is completely regular if, given any point \ensuremath{x} and closed set $F$ in \ensuremath{X} such
that \ensuremath{x} does not belong to $F$, they are separated by a continuous function, i.e.
$$
\{x\} \longrightarrow \ensuremath{X} \,\rightthreetimes\, [0,1] \cup \{F\} \longrightarrow \{x{\small\searrow}F\}
$$ where points $F$ and $1$ are topologically indistinguishable, $[0,1]$ goes to $x$,
and $F$ goes to $F$.
Every
completely regular space is also regular.
\item[] \ensuremath{X} is Tychonoff, or T3$\frac12$, completely T3, or completely regular Hausdorff, if
it is both T0 and completely regular.[2] Every Tychonoff space is both regular
Hausdorff and completely Hausdorff.
\item[] \ensuremath{X} is normal if any two disjoint closed subsets of \ensuremath{X} are separated by
neighbourhoods, i.e.
$$ \emptyset \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{x{\small\swarrow}x'{\small\searrow}X{\small\swarrow}y'{\small\searrow}y\} \longrightarrow \{x{\small\swarrow}x'=X=y'{\small\searrow}y\}
$$
In fact, by Urysohn lemma a space is normal if and only if any two disjoint
closed sets can be separated by a continuous function, i.e.
$$ \emptyset \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{0'\} \cup [0,1] \cup \{1'\} \longrightarrow \{0=0'{\small\searrow}x{\small\swarrow}1=1'\} $$
where points
$0',0$ and $1,1'$ are topologically indistinguishable,
$[0,1]$ goes to $x$, and both $0,0'$ map to point $0=0'$, and both $1,1'$ map to point
$1=1'$.
\item[] \ensuremath{X} is normal Hausdorff, or T4, if it is both T1 and normal. Every normal
Hausdorff space is both Tychonoff and normal regular.
\item[] \ensuremath{X} is completely normal if any two separated sets $A$ and $B$ are separated by
neighbourhoods $U\supset A$ and $V\supset B$
such that $U$ and $V$ do not intersect, i.e.????
$$\emptyset \longrightarrow \ensuremath{X} \,\rightthreetimes\, \{X{\small\swarrow}A\leftrightarrow U{\small\searrow}U'{\small\swarrow}W{\small\searrow}V'{\small\swarrow}V\leftrightarrow B{\small\searrow}X\} \longrightarrow \{U=U',V'=V\}$$
Every completely normal space is also normal.
% $$
% \emptyset \longrightarrow \ensuremath{X} \,\rightthreetimes\, \\ \{X{\small\swarrow}x\leftrightarrow U{\small\searrow}U'{\small\swarrow}W{\small\searrow}V'{\small\swarrow}V\leftrightarrow y{\small\searrow}X\} \longrightarrow
%\{X{\small\swarrow}x\leftrightarrow U=U'{\small\swarrow}W{\small\searrow}V'=V\leftrightarrow y{\small\searrow}X\} $$
% or in short
\item[] \ensuremath{X} is perfectly normal if any two disjoint closed sets are precisely
separated by a continuous function, i.e.
$$ \emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, [0,1]\longrightarrow \{0{\small\swarrow}X{\small\searrow}1\}
$$ where $(0,1)$ goes to the open point $X$, and $0$ goes to $0$, and $1$ goes to $1$.
Every perfectly normal space is also
completely normal.
\item[] \ensuremath{X} is extremally disconnected if the closure of every open subset of $X$ is open, i.e.
$$\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{U{\small\searrow}Z',Z{\small\swarrow}V\}\longrightarrow \{U{\small\searrow}Z'=Z{\small\swarrow}V\}$$
or equivalently
$$\emptyset\longrightarrow \ensuremath{X} \,\rightthreetimes\, \{U{\small\searrow}Z',Z{\small\swarrow}V\}\longrightarrow \{Z'=Z\}$$
\end{itemize}
\newpage
\section{Appendix B. Quotations from sources.}
For reader's convenience we quote here from the several sources we use.
[Bourbaki, General Topology, I\S10.2, Thm.1(d), p.101]:
\begin{quote} THEOREM I. Let $f: X\longrightarrow Y$ be a continuous mapping. Then the following
four statements are equivalent:
\begin{itemize}
\item[a)] \ensuremath{f} is proper.
\item[b)] \ensuremath{f} is closed and $f^{-1} (y)$ is quasi-compact for each $y\in Y$.
\item[c)] If $\mathfrak F$
is a filter on \ensuremath{X} and if $y \in Y$ is a cluster point of $f (\mathfrak F)$ then there
is a cluster point \ensuremath{x} of
such that $f (x) = y$.
\item[d)] If $\mathfrak U$ is an ultrafilter on \ensuremath{X} and if $y \in Y$ is a limit point of the ultrafilter
base $f (\mathfrak U)$, then there is a limit point \ensuremath{x} of $\mathfrak U$ such that $f (x) = y$.
\end{itemize}
\end{quote}
[Engelking, 3.2.1,p.136] (``compact'' below stands for ``compact Hausdorff''):
\begin{quote} 3.2.1. THEOREM. Let $A$ be a dense subspace of a topological space \ensuremath{X} and \ensuremath{f} a continuous
mapping of $A$ to a compact space $Y$. The mapping \ensuremath{f} has a continuous extension over \ensuremath{X} if and
only if for every pair $B_1,B_2$ of disjoint closed subsets of \ensuremath{Y} the inverse images $f^{-1}(B_1)$
and $f^{-1}(B_2)$ have disjoint closures in the space $X$.
\end{quote}
%
[Hausdorff, Set theory, \S40, p.259] (``$\varepsilon$'' stands for ``$\in$'', and
``$ U_x V_x$'' stands for ``$ U_x \cap V_x$'') :
\begin{quote}
From
the theorems about open sets we derive the following properties of the
neighborhoods:
\begin{itemize}
\item[(A)]Every point $x$ has at least one neighborhood $U_x$; and $U_x$ always
contains $x$.
\item[(B)]For any two neighborhoods $U_x$ and $V_x$ of the same point, there
exists a third, $W_x \leq U_x V_x$.
\item[(C)]Every point $y \varepsilon U_z$ has a neighborhood $U_y \leq U_x$.
\end{itemize}
It is now again possible to treat neighborhoods as unexplained
concepts and to use them as our starting point, postulating Theorems (A),
(B), and (C) as neighborhood axioms$.^1$ Open sets $G$ are then defined
as sums of neighborhoods or as sets in which every point $x \varepsilon G$ has a
neighborhood $U_x \leq G$ (the null set included). Theorems (1), (2), and
(3) about open sets are then provable.
....
$\\{\\}^1$ Such a program was carried through in the first edition of this book. [Grund-
z\"ugeder Mengenlehre. (Leipzig, 1914; repr. New York, 1949),]
\end{quote}
\newpage
\section{Appendix C. Examples of lifting properties in other categories.}
Here we give a list of elementary notions in algebra defined by lifting properties.
\begin{claim}\begin{itemize}
\item[ (i)] $(\emptyset\longrightarrow \{*\})^r$, $(0\longrightarrow R)^r$, and $\{0\longrightarrow \ZZ\}^r$ are the classes of surjections in
in tha categories of Sets, R-modules, and Groups, resp.,
(where $\{*\}$ is the one-element set, and in the category of (not necessarily abelian) groups, $0$ denotes the trivial group)
\item[ (ii)] $(\{\star,\bullet\}\longrightarrow \{*\})^l=(\{\star,\bullet\}\longrightarrow \{*\})^r$, $(R\longrightarrow 0)^r$, $\{\ZZ\longrightarrow 0\}^r$ are the classes of
injections in the categories of Sets, R-modules, and Groups, resp
\item[ (iii)] in the category of R-modules, \\
\ \ a module \ensuremath{P} is projective iff $0\longrightarrow P$ is in $(0\longrightarrow R)^{rl}$\\
\ \ a module I is injective iff $I\longrightarrow 0$ is in $(R\longrightarrow 0)^{rr}$
\item[ (iv)] in the category of Groups, \begin{itemize}
\item[] a finite group \ensuremath{H} is nilpotent iff $H\longrightarrow H\times H$ is in $\{\, 0\longrightarrow \ensuremath{G} : G\text{ arbitrary} \}^{lr}$
\item[] a finite group \ensuremath{H} is solvable iff $0\longrightarrow H$ is in $\{\, 0\longrightarrow A : A\text{ abelian }\}^{lr}= \{\, [G,G]\longrightarrow \ensuremath{G} : G\text{ arbitrary }\}^{lr}$
\item[] a finite group \ensuremath{H} is of order prime to $p$ iff $H\longrightarrow 0$ is in $\{\ZZ/p\ZZ\longrightarrow 0\}^r$
\item[] a finite group \ensuremath{H} is a p-group iff $H\longrightarrow 0$ is in $\{\ZZ/p\ZZ\longrightarrow 0\}^{rr}$
\item[] a group \ensuremath{H} is torsion-free iff $0\longrightarrow H$ is in $\{ n\ZZ\longrightarrow \ZZ: n>0 \}^r$
\item[] a group \ensuremath{F} is free iff $0\longrightarrow F$ is in $\{0\longrightarrow \ZZ\}^{rl}$
\item[] a homomorphism $f$ is split iff $f \in \{\, 0\longrightarrow \ensuremath{G} : G\text{ arbitrary} \}^r$
%+ a subgroup A is pure in a group $H$ iff $A\longrightarrow H$ is in $\{ n\ZZ\longrightarrow \ZZ : n>0 \}^r$
\end{itemize}
\end{itemize}
\end{claim}
{\bf
Proof.
}
In (iv), we use that a finite group $H$ is nilpotent iff the diagonal $\{
(h,h) : \ensuremath{h } \in \ensuremath{H} \}$ is subnormal in $H\times H$.
Other examples are straightforward to verify using the definitions.
\qed
%References:
\begin{thebibliography}{10}
\bibitem[Alexandroff]{Alexandroff}
\newblock Alexandroff, P. S.
\newblock Results in Topology in the last 25 years, Russian Math. Surveys, 15 (1960)
\newblock http://www.mathnet.ru/php/archive.phtml?wshow=paper\\jrnid=rm\&paperid=6719\&option\_lang=eng
\bibitem[BaysQuilder]{BaysQuilder}
\newblock Martin Bays.
\newblock A diagram chaser. Abandoned code for diagram chasing in a posetal model category.
\newblock \hhref{http://mishap.sdf.org/quilder/}
\bibitem[Bourbaki]{Bourbaki}
\newblock Nicolas Bourbaki.
\newblock General Topology. I\S10.2, Thm.1(d), p.101 (p.106 of file)
%\newblock http://mishap.sdf.org/mints-lifting-property-as-negation/tmp/Bourbaki\_General\_Topology.djvu
\bibitem[Engelking]{Engelking}
\newblock Ryszard Engelking.
\newblock General Topology. Thm.3.2.1, p.136
\bibitem[Gavrilovich, Lifting Property]{Gavrilovich, Lifting property}
\newblock Misha Gavrilovich.
\newblock Expressive power of the lifting property in elementary mathematics. A draft, current version.
\newblock \hhref{http://mishap.sdf.org/by:gavrilovich/expressive-power-of-the-lifting-property.pdf}. Arxiv arXiv:1707.06615 (7.17)
\bibitem[Gavrilovich, Elementary Topology]{Gavrilovich, Elementary Topology}
\newblock Misha Gavrilovich.
\newblock Elementary general topology as diagram chasing calculations with finite categories. A draft of a research proposal.
\newblock \hhref{http://mishap.sdf.org/by:gavrilovich/mints-elementary-topology-as-diagram-chasing.pdf}
\bibitem[Gavrilovich, Tame Topology]{Gavrilovich, Tame Topology}
\newblock Misha Gavrilovich.
\newblock Tame topology: a naive elementary approach via finite topological spaces. A draft of a research proposal.
\newblock \hhref{http://mishap.sdf.org/by:gavrilovich/zhenya-tame-topology.pdf}
\bibitem[Gavrilovich, DMG]{Gavrilovich, DMG}
\newblock Misha Gavrilovich,
\newblock Point set topology as diagram chasing computations. Lifting properties as intances of negation.
\newblock The De Morgan Gazette \ensuremath{5} no.~4 (2014), 23--32, ISSN 2053-1451
\newblock \href{http://mishap.sdf.org/by:gavrilovich/mints-lifting-property-as-negation-DMG_5_no_4_2014.pdf}{http://mishap.sdf.org/by:gavrilovich/mints-lifting-property-as-negation-DMG\_5\_no\_4\_2014.pdf}
\bibitem[Gavrilovich, A homotopy approach to set theory]{Gavrilovich, A homotopy approach to set theory}
\newblock Misha Gavrilovich.
\newblock A homotopy approach to set theory.
\newblock 15pp. \hhref{http://mishap.sdf.org/by:gavrilovich/what:work-in-progress/blat.pdf} \hhref{https://arxiv.org/abs/1006.4647}
\bibitem[GavrilovichHasson]{GavrilovichHasson}
\newblock Misha Gavrilovich, Assaf Hasson.
\newblock Exercises de style: A homotopy theory for set theory. Part I and II.
\newblock \href{http://mishap.sdf.org/by:gavrilovich-and-hasson/what:a-homotopy-theory-for-set-theory/Exercises_de_style_A_homotopy_theory_for_set_theory-I-II-IJM.pdf}{http://mishap.sdf.org/by:gavrilovich-and-hasson/what:a-homotopy-theory-for-set-theory/Exercises\_de\_style\_A\_homotopy\_theory\_for\_set\_theory-I-II-IJM.pdf}
\bibitem[GLZ]{GLZ}
\newblock Misha Gavrilovich, Alexandre Luzgarev, Vladimir Sosnilo.
\newblock A decidable fragment of diagram chasing without automorphisms.
\newblock preprint.
\newblock \hhref{http://mishap.sdf.org/mints-a-decidable-fragment-of-category-theory-without-automorphisms.pdf}
\bibitem[Gromov, Ergobrain]{Gromov, Ergobrain}
\newblock Misha Gromov.
\newblock Structures, Learning and Ergosystems: Chapters 1-4, 6.
\newblock December 30, 2011.
\newblock \hhref{http://www.ihes.fr/~gromov/PDF/ergobrain.pdf}
\bibitem[Gromov, Memorandum Ergo]{Gromov, Memorandum Ergo}
\newblock Misha Gromov.
\newblock Memorandon Ergo. October 29, 2015.
\newblock \hhref{http://www.ihes.fr/~gromov/PDF/ergo-cut-copyOct29.pdf}
\newblock Russian translation in Gromov M., Kolco tain: vselennaya, matematika, mysl'.
\newblock Per. \ensuremath{s} angl.yaz. N.B.Tsilevich. M.: MCNMO, 2017. 289 c. ISBN 978-5-4439-1117-5.
\bibitem[Hausdorff]{Hausdorff}
\newblock Felix Hausdorff.
\newblock Set theory. \S40,p.259.
\newblock 1962.
\newblock (translation of the third (1937) edition of MENGENLEHRE by Felix Hausdorff).
\newblock See also the Russian translation (1936) ed. P.S.Alexandroff and A.N.Kolmogorov, \S22.IX,p.113,
and the first (1914) German edition.
\bibitem[Hocking, Young]{Hocking, Young}
\newblock John~G.~Hocking, Gail~S.~Young.
\newblock Topology.
\newblock 1961
\bibitem[Taimanov]{Taimanov}
\newblock A. D. Taimanov.
\newblock On extension of continuous mappings of topological spaces.
\newblock Mat. Sb. (N.S.), 31(73):2 (1952), 459-463
\end{thebibliography}
\end{document}