Affine systems of equations and counting infinitary logic

We study the definability of constraint satisfaction problems (CSPs) in various fixed-point and infinitary logics. We show that testing the solvability of systems of equations over a finite Abelian group, a tractable CSP that was previously known not to be definable in Datalog, is not definable in the infinitary logic with finitely many variables and counting. This implies that it is not definable in least fixed-point logic or its extension with counting. We relate definability of CSPs to their classification obtained from tame congruence theory of the varieties generated by the algebra of polymorphisms of the template structure. In particular, we show that if this variety admits either the unary or affine type, the corresponding CSP is not definable in the infinitary logic with counting. (c) 2009 Elsevier B.V. All rights reserved.