Is there a complete set of topological properties for a topological space

general-topology

A property $P$ of topological spaces is called a topological property if for any two topological spaces $X$ and $Y$ which are homeomorphic, $X$ has property $P$ if and only if $Y$ has property $P$. Topological properties are useful because if one space has a certain topological property and another space does not, then the two spaces are not homeomorphic.

But I’d like to go in the other direction, namely showing that if two spaces have certain topological properties in common, then they are homeomorphic. My question is, for a topological space $X$, does there exist a nontrivial collection of topological properties such that any space which has all these properties is homeomorphic to $X$?

By nontrivial I mean excluding properties like “homeomorphic to $X$”.

Best Answer

For some classical spaces in topology there has been a long research tradition of finding nice unique characterisations of a space, i.e. a finite list of "real" properties (directly defined in terms of open sets and set theory, like compactness and connectedness (and their local versions) and various ones) such that a space that satisfies that finite list of properties is homeomorphic to the space in question. To be useful, it should be non-circular ones (so being a simple-closed curve is not a "good" property to characterise the circle, because that's already defined as being homeomorphic to a circle, basically).

For zero-dimensional spaces we have the classic ones for the Cantor set (the unique zero-dimensional compact metrisable space without isolated points), the rationals (the only countable metrisable space without isolated points), the irrationals (the only completely metrisable separable zero-dimensional, nowhere locally compact space) and a few more.

For connected metric spaces (often continua) we have classic characterisations of $[0,1]$ (metrisable Peano-continuum with exactly two non-cutpoints), $\mathbb{R}$ (separable metrisable, connected, locally connected and every point a strong cut-point (exactly two components of the space minus that point)), $S^1$( metrisable Peano continuum, with no cutpoints and such that every pair is a cutset) and a few others.

For infinite-dimensional spaces we have the topological classification of completely metrisable separable vector spaces (they're all homeomorphic, so purely topologically there is no difference between $C([0,1])$ and separable Hilbert space, e.g.)

But for general spaces there is no hope of such results, they are just too wild and have lots of connections with complicated set theory. By transfinite recursion one can sometimes construct non-homeomorphic spaces of very similar type (so that they'd be the same for a list of standard properties, at least that I could think of (I did this kind of thing in my PhD thesis) but that nonetheless are non-homeomorphic because they are constructed that way: by a diaginalisation argument one can often enumerate in advance all possible candidate functions, and then during the construction ensure that none of them will be a homeomorphism of the eventually resulting space; it's a common trick topologists use, Sierpinski already did it back in the 1920's). So sometimes spaces are not homeomorpjic because they aren't by construction. And if I made $2^{\aleph_1}$ distinct spaces in some construction, there is no way I could distinguish them even using a finite list from $\aleph_1$ properties, if could invent that many "properties" in the first place.. The only option left would be to give up and allow "homeomorphic to $X$" as a (useless) property for all $X$.

Just my thoughts..