Let $X$ be a finite simplicial complex and let $B$ denote the set of barycenters of the simplices of $X$. [McCord](http://projecteuclid.org/download/pdf_1/euclid.dmj/1077376525) constructed a $T\_0$ topology on $B$ with the property that the inclusion $B \to X$ is a weak homotopy equivalence. [Clader](http://www.math.columbia.edu/~lipshitz/teaching/CladerThesis.pdf) showed that the geometric realization of $X$ can be recovered up to homeomorphism by iterating McCord's construction on the $n$th barycentric subdivision of $X$, taking an inverse limit, and passing to a certain quotient space. Moreover she proved that the inverse limit deformation retracts onto the quotient space and thus every finite simplicial complex is homotopy equivalent to an inverse limit of finite spaces.


This is kind of a shocking result (though the shock wears off quickly once you stop and think about it), and it leads me to wonder:



> 
> Would it be possible to dispense with the language of simplicial complexes entirely and rewrite the foundations of homotopy theory using finite topological spaces and their inverse limits?
> 
> 
> 


I am mainly interested in whether or not this is *possible*, though you may feel free to include in your answer remarks about whether or not this is *desirable*. Some possible complications that come to mind:


* Many important topological spaces (e.g. classifying spaces for most groups) do not have the homotopy type of a finite simplicial complex.
* One does not directly have access to certain tools, such as a Morse theory.


Perhaps one can hope to approximate whatever is lost using finite models?



How much of homotopy theory can be done using only finite topological spaces?
Vidit, thanks for the advertisement; Paul I'll answer your email shortly.
As a minor point, there is a small but subtle mistake in Clader's work 
that is corrected in [Matthew Thibault's 2013 Chicago thesis](http://search.proquest.com/docview/1424274072), which goes
further in that direction. 


I do intend to finish the advertised book, but it is too incomplete to 
circulate yet. There is actually a large and interesting picture that
connects mainstream algbraic topology to combinatorics via finite spaces.  

However, the right level 
of generality is $T\_0$-Alexandroff spaces, $A$-spaces for short. These
are topological spaces in which arbitrary rather than just finite
intersections of open sets are open, and of course finite $T\_0$-spaces
are the obvious examples. One can in principle answer Paul's question in the 
affirmative, but the finiteness restriction feels artificial and the connection
between $A$-spaces and simplicial complexes is far too close to ignore. 


The category of $A$-spaces is isomorphic to the category of posets, $A$-spaces 
naturally give rise to ordered simplicial complexes (the order complex of a 
poset) and thus to simplicial sets, while abstract simplicial complexes naturally 
give rise to $A$-spaces (the face poset).
Subdivision is central to the theory, and barycentric subdivision of a 
poset is WHE to the face poset of its order complex. 
Categories connect up since the second subdivision of a category is
a poset, which helps illuminate Thomason's equivalence between the
homotopy categories of $\mathcal{C}at$ and $s\mathcal{S}et$. 


Weak and actual homotopy equivalences are wildly different for 
$A$-spaces. In the usual world of spaces, they correspond to 
homotopy equivalences and simple homotopy equivalences, respectively,
a point of view that Barmak's book focuses on. 
The $n$-sphere is WHE to a space with $2n+2$ points, and that is the 
minimum number possible. 


If the poset $\mathcal{A}\_pG$ of non-trivial elementary abelian $p$-subgroups of a 
finite group $G$ is contractible, then $G$ has a normal $p$-subgroup. A celebrated conjecture of Quillen says in this language that if 
$\mathcal{A}\_pG$ is weakly contractible (WHE to a point), then it is 
contractible and hence $G$ has a normal $p$-subgroup. There are many 
interesting contractible finite spaces that are not weakly contractible. 


These facts just scratch the surface and were nearly all previously known, 
but there is much that is new in the book, some of it due to students
at Chicago where I have taught this material in our REU off and on since 2003. 
This is ideal material for the purpose. (Obsolete notes and even current ones 
can be found on my web page by those sufficiently interested to search: Minian,
Barmak's thesis advisor in Buenos Aires, found them there and started off work
in Argentina based on them.) I apologize for this extended advertisement,
but perhaps Paul's question gives me a reasonable excuse.


