Ultrafilter unions: an exercise in modal definability
Yde Venema, A. Nepomuceno, José Francisco Quesada, F.J. Salguero · 2000
We introduce the notion of the ultrafilter union of a family of pointed modal models over an ultrafilter of the index set and we argue that this operation is the modal analogue of taking ultraproducts in the model theory of first-order logic. We use this notion to give a structural characterization of the modally definable classes of pointed models. 1