Boolean Algebras Canonically Defined - Boolean Homomorphisms

Boolean Homomorphisms

A Boolean homomorphism is a function h: A→B between Boolean algebras A, B such that for every Boolean operation mf_i,

h(mf_i(x₀,…,x_m−1)) = mf_i(h(x₀),…,h(x_m−1)).

The category Bool of Boolean algebras has as objects all Boolean algebras and as morphisms the Boolean homomorphisms between them.

There exists a unique homomorphism from the two-element Boolean algebra 2 to every Boolean algebra, since homomorphisms must preserve the two constants and those are the only elements of 2. A Boolean algebra with this property is called an initial Boolean algebra. It can be shown that any two initial Boolean algebras are isomorphic, so up to isomorphism 2 is the initial Boolean algebra.

In the other direction, there may exist many homomorphisms from a Boolean algebra B to 2. Any such homomorphism partitions B into those elements mapped to 1 and those to 0. The subset of B consisting of the former is called an ultrafilter of B. When B is finite its ultrafilters pair up with its atoms; one atom is mapped to 1 and the rest to 0. Each ultrafilter of B thus consists of an atom of B and all the elements above it; hence exactly half the elements of B are in the ultrafilter, and there as many ultrafilters as atoms.

For infinite Boolean algebras the notion of ultrafilter becomes considerably more delicate. The elements greater or equal than an atom always form an ultrafilter but so do many other sets; for example in the Boolean algebra of finite and cofinite sets of integers the cofinite sets form an ultrafilter even though none of them are atoms. Likewise the powerset of the integers has among its ultrafilters the set of all subsets containing a given integer; there are countably many of these "standard" ultrafilters, which may be identified with the integers themselves, but there are uncountably many more "nonstandard" ultrafilters. These form the basis for nonstandard analysis, providing representations for such classically inconsistent objects as infinitesimals and delta functions.