Goldblatt-Thomason theorem is a theorem in modal logic which characterizes the elementary classes which are also modally definable classes. It says that if is an elementary class of Kripke frames, then is modally definable iff is closed under taking bounded morphic images, generated subframes, disjoint unions, and reflects ultrafilter extensions.
The theorem is based on the duality between the category of Boolean algebras and Sets
where is the power set functor and assigns to a Boolean algebra the set of its ultrafilters. is left-adjoint to , but in general they do not form a dual equivalence of categories. This only holds in the finite case, where it is enough for to assign to a Boolean algebra the set of its atoms.
The idea of the proof is to translate between Kripke frames and Modal algebras, and obtain the above result by an application of Birkhoff's HSP theorem.
Last revised on March 22, 2017 at 15:13:36. See the history of this page for a list of all contributions to it.