We study digraphs preserved by a Maltsev operation, Maltsev digraphs. We show that these digraphs retract either onto a directed path or to the disjoint union of directed cycles, showing that the… (More)

THOMAS R. SHULTZ∗,†,¶, FRANÇOIS RIVEST‡,‖, LÁSZLÓ EGRI†,∗∗, JEAN-PHILIPPE THIVIERGE§,†† and FRÉDÉRIC DANDURAND∗,‡‡ ∗Department of Psychology, McGill University, 1205 Penfield Avenue, Montreal, QC H3A… (More)

We introduce symmetric Datalog, a syntactic restriction of linear Datalog and show that its expressive power is exactly that of restricted symmetric Krom monotone SNP. The deep result of Reingold… (More)

The Dichotomy Conjecture for constraint satisfaction problems (CSPs) states that every CSP is in P or is NP-complete (Feder-Vardi, 1993). It has been verified for conservative problems (also known as… (More)

The minimum unsatisfiability version of a constraint satisfaction problem (MinCSP) asks for an assignment where the number of unsatisfied constraints is minimum possible, or equivalently, asks for a… (More)

We show that the directed st-connectivity problem cannot be expressed in symmetric Datalog, a fragment of Datalog introduced in [5]. It was shown there that symmetric Datalog programs can be… (More)

In the deletion version of the list homomorphism problem, we are given graphs G and H, a list $$L(v)\subseteq V(H)$$ L ( v ) ⊆ V ( H ) for each vertex $$v\in V(G)$$ v ∈ V ( G ) , and an integer k.… (More)