abstract duality: opposite category,
Perhaps the most general duality falling under this heading is that between locales (on the space side) and frames (on the quantity side). Of course, this duality is not very deep at all; the category Loc of locales is simply defined to be the opposite of the category Frm of frames. But there are several interesting dualities between subcategories of these.
Stone duality is often described for topological spaces rather than for locales. In this case, the most general duality is that between sober spaces and frames with enough points (which correspond to topological locales). In many cases, one requires the ultrafilter theorem (or other forms of the axiom of choice) in order for the duality to hold when applied to topological spaces, while the duality holds for locales even in constructive mathematics.
Any distributive lattice generates a free frame. The locales which arise in this way can be characterized as the coherent locale?s, and this gives a duality between distributive lattices and coherent locales. Note that one must additionally restrict to “coherent maps” between coherent locales. Also, at least assuming the axiom of choice, every coherent locale is topological, so we may say “coherent space” instead.
The duality which is due to Marshall Stone, and which gives its name to the subject, is the duality between Stone spaces and Boolean algebras. Specifically, a distributive lattice is a Boolean algebra precisely when the free frame it generates is the topology of a Stone space, and any continuous map of Stone spaces is coherent. Therefore, the category of Stone spaces is dual to the category of Boolean algebras. The Boolean algebra corresponding to a Stone space consists of its clopen sets.
One way of explaining this classical Stone duality is via the following sequence of equivalences of categories
Note that a finite Stone space is necessarily discrete, and these correspond to the finite Boolean algebras, i.e. . However, since Boolean algebras form a locally finitely presentable category, we have (see ind-object and pro-object). In consequence, : i.e. Stone spaces are equivalent to profinite sets, in this context then often called profinite spaces.
If is a Lawvere theory on , we can talk about Stone -algebras, i.e. -algebras with a compatible Stone topology, and compare the resulting category with the category of pro-(finite -algebras). The previous duality says that these categories are equivalent when is the identity theory. It is also true in many other cases, such as:
However it is false for some , such as:
All of these can be found in chapter VI of Johnstone’s book cited below.
The corresponding fact is also notably false for groupoids, i.e. is not equivalent to , in contrast to the case for groups. (Of course, groupoids are not described by a Lawvere theory.)
is all about Stone duality.
There is a version in model theory, Makkai duality,
Another variant is in
Discussion in E-∞ geometry is in