Abstract

We are concerned with modal logics in the class EM0 of extensions of M0 . G denotes re exive frames. MG the modal logic on G in the sense of Kripke. M is nite if M = MG for some nite G. Finite G's will be drawn as framed diagrams, e.g. G = ! ; G = ! ; the latter shorter denoted by . EM0 is a complete lattice with zero M0 and one M . If M M0 M0 is a succ of M. An ip of M is an immediate predecessor of M. E.g. M ! and M are the only ip's of M in ES4. M[P] denotes the extension of M adding P as a \new axiom", e.g. S4 = M0 [p ! p]. M is nitely axiomatizable if M = M0 [P] for some formula P. One easily shows if M is f.a. then each predecessor is separated from M by an ip of M. It is known that each nite M is f.a. and has only nitely many succ's all of which are nite