Subject: categories: Abelian Topological Groups

From: Bill Rowan <[email-masked]>
Date: Mon, 30 Apr 2001 21:37:18 -0700 (PDT)

I am attempting to construct the ideal abelian category within which live
complete, hausdorff abelian topological groups.  The idea is that the
quotients of such a group, in the abelian category, would be completions
of the group with respect to topologies coarser than the given one.  The
subobjects would be those topologies.  Of course, having a topology as an
object in the abelian category means we have to have objects in the category
other than abelian groups.

Of course I want to know if this has been done before.  Also, what other ideas
are there about the ideal abelian category containing these groups?  Mac Lane
felt that compactly-generated spaces formed the ideal base category for
topological algebra.  I seem to be using the category of complete, hausdorff
uniform spaces as a base category.  I wrote a paper on (universal) algebras
with a compatible uniformity, and got some nice results about the congruence
(actually, uniformity) lattices.  But, admittedly, algebras with compatible
uniformities have drawbacks as a foundation for topological algebra because
even something like the complex numbers cannot be formalized as such, the
multiplication not being uniformly continuous.

Bill Rowan

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From: Michael Barr <[email-masked]>
Date: Wed, 2 May 2001 06:11:51 -0400 (EDT)

One thing is clear: your ideal abelian category is not abelian.
Furthermore, you don't get to choose your sub- and quotient objects; they
are imposed by the category.  Moreover, although a weaker topology (or an
abelian group with a weaker topology, which is what I assume is meant) is
certainly a subobject, it is not regular, which every subobject in an
abelian category must be.  In fact, the only abelian categories of
topological abelian groups I am aware of are the discrete groups and the
dual category of compact groups.  For me, the ideal category of
topological abelian groups is SP(LCA), the subobjects of products of
locally compact abelian groups.  It is not abelian, but it is
*-autonomous. It is equivalent to the category of weakly topologized
abelian groups or SP(R/Z), subobjects of powers of the circle.