On notions of representability for cylindric-polyadic algebras, and a solution to the finitizability problem for quantifier logics with equality

摘要

We consider countable so-called rich subsemigroups of ((omega)omega,circle); each such semigroup T gives a variety CPEA(T) that is axiomatizable by a finite schema of equations taken in a countable subsignature of that of omega-dimensional cylindric-polyadic algebras with equality where substitutions are restricted to maps in T. It is shown that for any such T, U is an element of CPEA(T) if and only if A is representable as a concrete set algebra of omega-ary relations. The operations in the signature are set-theoretically interpreted like in polyadic equality set algebras, but such operations are relativized to a union of cartesian spaces that are not necessarily disjoint. This is a form of guarding semantics. We show that CPEA(T) is canonical and atom-canonical. Imposing an extra condition on T, we prove that atomic algebras in CPEA(T) are completely representable and that CPEA(T) has the super amalgamation property. If T is rich and finitely represented, it is shown that CPEA(T) is term definitionally equivalent to a finitely axiomatizable Sahlqvist variety. Such semigroups exist. This can be regarded as a solution to the central finitizability problem in algebraic logic for first order logic with equality if we do not insist on full fledged commutativity of quantifiers. The finite dimensional case is approached from the view point of guarded and clique guarded (relativized) semantics of fragments of first order logic using finitely many variables. Both positive and negative results are presented. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim