Seminars

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2017
Fri 10th Nov
10:00 am
11:00 am
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Momar Sakho: "Complexity of Certain Query Answering on Hyperstreams"
A hyperstream is a sequence of streams with references to others. We study the complexity of computing certain answers for queries defined by automata and evaluated on hyperstreams of words. We show that the problem is PSPACE-complete for deterministic query automata, but that it can be solved in PTime for linear hyperstreams even with factorization.
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Salle B21
Fri 3rd Nov
10:30 am
12:00 pm
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Joanna Ochremiak, Paris 7: "Proof complexity of constraint satisfaction problems"
Many natural computational problems, such as satisfiability and
systems of equations, can be expressed in a unified way as constraint
satisfaction problems (CSPs). In this talk I will show that the usual
reductions preserving the complexity of the constraint satisfaction
problem preserve also its proof complexity. As an application, I will
present two gap theorems, which say that CSPs that admit small size
refutations in some classical proof systems are exactly the constraint
satisfaction problems which can be solved by Datalog.

This is joint work with Albert Atserias.

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B21
Fri 13th Oct
11:00 am
1:00 pm
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Dimitri Gallois: On parallel rewriting
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B21
Fri 29th Sep
10:00 am
12:00 pm
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Nicolas Bacquey: "An algorithm for deciding the equivalence of tree transducers"
As an extension of word transformations, tree transformations have numerous applications in computer science : XSLT transformations, Unix packages installation and removal, databases queries... Likewise, there are many formal models to describe these transformations. However, the proof of formal properies on these models is often difficult, or even undecidable.
In this talk, I will be interested in one of the simplest model for tree transformations, namely deterministic top-down tree transducers (DTOP). It has been known for a while that the equivalence problem of DTOPs can be solved via an earliest normal form comparison algorithm, that is in 2EXPTIME. However, when applying this algorithm to practical cases, it seemed that the worst case was not bound to happen often, if ever.
I will present a new algorithm for the problem, based on the search of counterexamples via the expansion and unification of a set of rules over states of DTOPs. The most interesting feature of this algorithm is that it runs in exponential time, thus proving that the equivalence problem of DTOPs is in fact EXPTIME-complete.

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Lille B31

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