Linear Datalog and Bounded Path Duality of Relational Structures

In this paper we systematically investigate the connections between logics with a finite number of variables, structures of bounded pathwidth, and linear Datalog Programs. We prove that, in the context of Constraint Satisfaction Problems, all these concepts correspond to different mathematical embodiments of a unique robust notion that we call bounded path duality. We also study the computational complexity implications of the notion of bounded path duality. We show that every constraint satisfaction problem \csp(\best) with bounded path duality is solvable in NL and that this notion explains in a uniform way all families of CSPs known to be in NL. Finally, we use the results developed in the paper to identify new problems in NL

Constraint Satisfaction Problems in Non-Deterministic Logarithmic Space

We study which constraint satisfaction problems (CSPs) are solvable in NL. In particular, we identify a general condition called bounded path duality, that explains all the families of CSPs previously known to be in NL. Bounded path dualit