Ultraproducts of fields, II June 28, 2008
Posted by Alexandre Borovik in Uncategorized.trackback
I continue my post on ultraproducts. So, we want to understand in what sense an ultraproduct of finite fields of unbounded order is a limit at infinity of finite fields. The answer now should be obvious: since ultraproducts are residue fields for maximal ideals in the cartesian product
the topology in question should be the canonical topology (Zariski topology) of the spectrum of the ring . It instantly follows from the description of ideals and maximal ideals in that this is the Stone topology on the set of ultrafilters on , or, what is th same, the Cech-Stone compactification of the set with descrete topology. Therefore the answer is: an ultraproduct is the limit in the Cech-Stone compactification of a discrete countable set.
I have to admit that at this point I reached limits of my knowledge of set-theoretic topology and had to dip into Wikipedia. It happened that , and even more so its non-principal part is characterised by some unique properties: If the continuum hypothesis holds then is the unique Parovicenko space, up to isomorphism.
According to Wikipedia, a Parovicenko space is a topological space X satisfying the following conditions:
- X is compact Hausdorff
- X has no isolated points
- X has weight c, the cardinality of the continuum (this is the smallest cardinality of a base for the topology).
- Every two disjoint open Fσ subsets of X have disjoint closures
- Every nonempty Gδ of X has non-empty interior.
As you can see, is uniquely characterised by very natural properties.
It is yet another manifestation of of one of the most pardoxical properties of mathematical infinity: canonicity of workable constructions in the infinite domain.
I wonder how much of what is natural about can be put in category theoretic terms. The you mention is a functor, Stone-Cech compactification, from Top (the category of topological spaces) to CHaus (the category of compact Hausdorff spaces). It is left adjoint to the underlying functor going the other way.
So, . In the case of a map to a compact Hausdorff space is determined by a map from to that space.
But does the formation of by removing the principal part, correspond to a category theoretic construction?