We propose a proof-theoretic approach for gaining evidence that certain parameterized problems are not fixed-parameter tractable. We consider proofs that witness that a given propositional formulaâ€¦ (More)

We extend the results of 8] to Jackson networks. Each node j, 1 j J of the network consists of N identical channels, each with an innnite buuer and a single server with service rate j. The network isâ€¦ (More)

We consider a Jackson-type network, each of whose nodes contains N identical channels with a single server. Upon arriving at a node, a task selects m of the channels at random, and joins the shortestâ€¦ (More)

We survey known results about the complexity of surjective homomorphism problems, studied in the context of related problems in the literature such as list homomorphism, retraction and compaction. Inâ€¦ (More)

We propose a proof-theoretic approach for gaining evidence that certain parameterized problems are not fixed-parameter tractable. We consider proofs that witness that a given propositional CNFâ€¦ (More)

For a connected graph G = (V,E), a subset U âŠ† V is called a disconnected cut if U disconnects the graph and the subgraph induced by U is disconnected as well. We show that the problem to test whetherâ€¦ (More)