Symmetric Linear Arc Monadic Datalog and Gadget Reductions
For researchers in finite-domain constraint satisfaction, this paper provides a complete classification of CSPs solvable by a restricted but important Datalog fragment, with decidable membership.
The paper characterizes the constraint satisfaction problems (CSPs) solvable by symmetric linear arc monadic (slam) Datalog programs, showing they correspond to those with a gadget reduction to a specific Boolean CSP. It provides exact characterizations via unfolded caterpillar duality and universal-algebraic conditions, and proves decidability of whether a finite-domain CSP can be expressed in slam Datalog.
A Datalog program solves a constraint satisfaction problem (CSP) if and only if it derives the goal predicate precisely on the unsatisfiable instances of the CSP. There are three Datalog fragments that are particularly important for finite-domain constraint satisfaction: arc monadic Datalog, linear Datalog, and symmetric linear Datalog, each having good computational properties. We consider the fragment of Datalog where we impose all of these restrictions simultaneously, i.e., we study \emph{symmetric linear arc monadic (slam) Datalog}. We characterise the CSPs that can be solved by a slam Datalog program as those that have a gadget reduction to a particular Boolean constraint satisfaction problem. We also present exact characterisations in terms of a homomorphism duality (which we call \emph{unfolded caterpillar duality}), and in universal-algebraic terms (using known minor conditions, namely the existence of quasi Maltsev operations and $k$-absorptive operations of arity $nk$}, for all $n,k \geq 1$). Our characterisations also imply that the question whether a given finite-domain CSP can be expressed by a slam Datalog program is decidable.