A formal framework is given for the characterizability of a class of belief revision operators, defined using minimization over a class of partial preorders, by postulates. It is shown that for partial orders characterizability implies a definability property of the class of partial orders in monadic second-order logic. Based on a non-definability result for a class of partial orders, an exampl...

We investigate definability in the set of isomorphism types of finite semilattices ordered by embeddability; we prove, among other things, that every finite semilattice is a definable element in this ordered set. Then we apply these results to investigate definability in the closely related lattice of universal classes of semilattices; we prove that the lattice has no non-identical automorphism...

We show that for every countable recursively saturated model M of Peano arithmetic and subset A⊆M, there exists a full satisfaction class SA⊆M2 such A is definable in (M,SA) without parameters. It follows model, which makes element definable, thus the expanded minimal rigid. On other hand, as observed by Roman Kossak, S are two elements have same arithmetical type, but exactly one them S. In pa...