Henrik Björklund

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We suggest the first strongly subexponential and purely combinatorial algorithm for solving the mean payoff games problem. It is based on iteratively improving the longest shortest distances to a sink in a possibly cyclic directed graph. We identify a new “controlled” version of the shortest paths problem. By selecting exactly one outgoing edge in each of(More)
We study the containment, satisfiability, and validity problems for conjunctive queries over trees with respect to a schema. We show that conjunctive query containment and validity are 2EXPTIMEcomplete w.r.t. a schema (DTD or Relax NG). Furthermore, we show that satisfiability for conjunctive queries w.r.t. a schema can be decided in NP. The problem is(More)
The complexity of containment and satisfiability of conjunctive queries over finite, unranked, labeled trees is studied with respect to the axes Child , NextSibling , their transitive and reflexive closures, and Following . For the containment problem a trichotomy is presented, classifying the problems as in PTIME, coNP-complete, or Π 2 -complete. For the(More)
We give a simple, direct, and constructive proof of memoryless determinacy for parity and mean payo& games. First, we prove by induction that the 8nite duration versions of these games, played until some vertex is repeated, are determined and both players have memoryless winning strategies. In contrast to the proof of Ehrenfeucht and Mycielski, Internat. J.(More)
The complexity of solving infinite games, including parity, mean payoff, and simple stochastic, is an important open problem in verification, automata, and complexity theory. In this paper, we develop an abstract setting for studying and solving such games, based on function optimization over certain discrete structures. We introduce new classes of(More)
Incremental view maintenance for XPath queries asks to maintain a materialized XPath view over an XML database. It assumes an underlying XML database <i>D</i> and a query <i>Q</i>. One is given a sequence of updates <i>U</i> to <i>D</i>, and the problem is to compute the result of <i>Q</i>(<i>U</i>(<i>D</i>)): the result of evaluating query <i>Q</i> on(More)
We prove that minimizing finite automata is NP-hard for almost all classes of automata that extend the class of deterministic finite automata. More specifically, we show that minimization is NP-hard for all finite automata classes that subsume the class of δNFAs which accept strings of length at most three. Here, δNFAs are the finite automata that are(More)