Title Canonicity and First-Order Completeness: wherein lies the difference?


If a modal logic is determined by a first-order definable class of frames, then it is valid in its canonical frames. The converse of this is known to fail for some logics.

The talk will present separate characterisations of these notions of canonicity and first-order completeness that traces their difference to the fact that the operation of forming the canonical frame of a modal algebra does not commute with the formation of ultrapowers.