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X-WR-CALNAME:GREYC UMR CNRS 6072 - Groupe de Recherche en Informatique, Image, et Instrumentation de Caen
X-ORIGINAL-URL:https://www.greyc.fr
X-WR-CALDESC:évènements pour GREYC UMR CNRS 6072 - Groupe de Recherche en Informatique, Image, et Instrumentation de Caen
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TZID:Europe/Paris
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DTSTART:20250330T010000
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DTSTART:20251026T010000
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DTSTART;TZID=Europe/Paris:20250114T104500
DTEND;TZID=Europe/Paris:20250114T114500
DTSTAMP:20260503T185010
CREATED:20250121T094507Z
LAST-MODIFIED:20250121T094507Z
UID:11774-1736851500-1736855100@www.greyc.fr
SUMMARY:Séminaire Algorithmique : Abdallah Saffidine (Univ. South New Wales\, Sydney\, Australie)\, « Generalizing Roberts’ characterization of unit interval graphs? How not to do it! »
DESCRIPTION:A celebrated result by Roberts [1969] establishes that an interval graph can be represented by unit-length intervals on the real line if and only if no vertex has three non-adjacent neighbors. In 2016\, Durán et al. conjectured a generalization of this result\, which we recently investigated. We ran an industrial mixed integer programming (MIP) solver on a cluster for several months\, looking for a counter-example to the conjecture. With no success in sight\, we decided another approach was needed. Going back to the drawing board was surprisingly successful: not only did we prove the conjecture to be true; but we also proved it to be false by exhibiting a counter-example that would have eluded our MIP solver for decades. This talk resolves this apparent inconsistency and can serve as a very gentle introduction to structural graph theory. \nBased on joint work with Virginia Ardévol Martínez\, Romeo Rizzi\, Florian Sikora\, and Stéphane Vialette\, MFCS 2024.
URL:https://www.greyc.fr/event/seminaire-algorithmique-abdallah-saffidine-univ-south-new-wales-sydney-australie-generalizing-roberts-characterization-of-unit-interval-graphs-how-not-to-do-it/
LOCATION:Sciences 3- S3 351
CATEGORIES:Amacc,General,Séminaire Algo
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20250121T104500
DTEND;TZID=Europe/Paris:20250121T114500
DTSTAMP:20260503T185010
CREATED:20250121T085857Z
LAST-MODIFIED:20250121T085937Z
UID:11772-1737456300-1737459900@www.greyc.fr
SUMMARY:Séminaire Algorithmique : Djamel Eddine Amir (LISN\, Univ. Paris-Saclay) « Computability of Compact Spaces »
DESCRIPTION:The topological properties of a set have a strong impact on its computability properties. A striking illustration of this idea is given by spheres\, closed manifolds and finite graphs without endpoints: if a set X is homeomorphic to a sphere\, a closed manifold or such a graph\, then any algorithm that semicomputes X in some sense can be converted into an algorithm that fully computes X. In other words\, the topological properties of X enable one to derive full information about X from partial information about X. In that case\, we say that X has computable type. Those results have been obtained by Miller and Iljazović and others in recent years. We give a characterization of finite simplicial complexes having computable type using a local property of stars of vertices. We argue that the stronger\, relativized version of computable type\, is better behaved. Consequently\, we obtain characterizations of strong computable type\, related to the descriptive complexity of topological invariants. We study two families of topological invariants of low descriptive complexity\, expressing the extensibility and the null-homotopy of continuous functions. We apply the theory to revisit previous results and obtain new ones. This leads us to investigate the expressive power of low complexity invariants in the Borel hierarchy and their ability to differentiate between spaces. Notably\, we show that they are sufficient for distinguishing finite topological graphs.
URL:https://www.greyc.fr/event/seminaire-algorithmique-djamel-eddine-amir-lisn-univ-paris-saclay-computability-of-compact-spaces/
LOCATION:Sciences 3- S3 351
CATEGORIES:Amacc,General,News,Séminaire Algo
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20250128T104500
DTEND;TZID=Europe/Paris:20250128T114500
DTSTAMP:20260503T185010
CREATED:20250127T162127Z
LAST-MODIFIED:20250127T162310Z
UID:11779-1738061100-1738064700@www.greyc.fr
SUMMARY:Séminaire Algorithmique : Geoffroy Caillat-Grenier (LIRMM\, Univ. Montpellier)\, « From combinatorics of graphs to communication complexity in algorithmic information theory »
DESCRIPTION:Several connexions between information theory and combinatorics of graphs have been explored in the last decades. Following this path\, we use tools from spectral graph theory to show impossibility results in information theoretic cryptography. We place ourselves in the Kolmogorov complexity framework. After a short introduction to algorithmic information theory and secret key agreement\, we will study some spectral and combinatorial properties of bipartite graphs explicitly constructed. \nThose graphs can represent the inputs for a communication problem. Spectral and combinatorial properties of these objects imply information  inequalities that make impossible some communication protocols for secret key agreement. For instance\, if the graph representing the inputs of the secret key agreement is a spectral expander\, we get the worst case in terms of communication complexity. Moreover\, the communication protocol is inherently uneven: for two participants\, only one of them needs to send all the needed information. Theses facts are connected to the mutual information unextractability phenomenon.
URL:https://www.greyc.fr/event/seminaire-algorithmique-geoffroy-caillat-grenier-lirmm-univ-montpellier-from-combinatorics-of-graphs-to-communication-complexity-in-algorithmic-information-theory/
LOCATION:Sciences 3- S3 351
CATEGORIES:Amacc,General,News,Séminaire Algo
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