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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:20230326T010000
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DTSTART:20231029T010000
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230201T140000
DTEND;TZID=Europe/Paris:20230201T150000
DTSTAMP:20260629T102025
CREATED:20230116T125741Z
LAST-MODIFIED:20230116T125741Z
UID:11062-1675260000-1675263600@www.greyc.fr
SUMMARY:Aurore Guillevic - Factoring RSA of 240 decimal digits and computing discrete logarithms in a 240-decimal-digit prime field with the same software and hardware.
DESCRIPTION:In December 2019 were announced two new record computations: the factorization of RSA-240 (240 digits\, 795 bits) and discrete logarithm computation in a prime field of the same size\, with the same software\, running on the same platforms. This is the first time that integer factorization (IF) and discrete logarithm (DL) of the same size are computed together. The previous RSA factorization record was in Dec. 2009 by Kleinjung et al.\, who factorized RSA-768 (bits\, 232 decimal digits). The previous DL record computation was in June 2016 by Kleinjung et al.\, for a prime field of 768 bits: there were seven years between RSA factorization and DL computation records of the same size\, and ten years between the two RSA factorization records. \nThe best known algorithm to address challenges of this size is the Number Field Sieve\, designed in the 90’s\, first for integer factorization\, then adapted to discrete logarithm computation. The free software Cado-NFS implements the NFS algorithm\, and has been developed for ten years. The same software modules were used\, with different parameters\, on four different computing resources in EU and US\, to achieve the two records. Thanks to algorithmic variants well-suited for large sizes\, and fine tuning of the parameters\, the DL record was actually three times faster than expected compared to the previous DL record\, when comparing on the same hardware. Moreover our work shows that computing a discrete logarithm is not much harder than a factorization of the same size. \nThe Number Field Sieve algorithm for integer factorization will be presented\, at a beginner’s level. Then I will present our algorithmic improvements\, some of the software properties\, and parameter options chosen for the records. Finally I will discuss on expectations on how the computations would scale for larger records. \n  \nVISIO BBB
URL:https://www.greyc.fr/event/aurore-guillevic-factoring-rsa-of-240-decimal-digits-and-computing-discrete-logarithms-in-a-240-decimal-digit-prime-field-with-the-same-software-and-hardware/
LOCATION:En distanciel
CATEGORIES:Séminaire Cryptologie et sécurité
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230202T140000
DTEND;TZID=Europe/Paris:20230202T153000
DTSTAMP:20260629T102025
CREATED:20230201T114800Z
LAST-MODIFIED:20230307T083457Z
UID:11087-1675346400-1675351800@www.greyc.fr
SUMMARY:Séminaire IMAGE : « Patch and attention for image editing » (Nicolas Cherel)
DESCRIPTION:Nous aurons le plaisir d’écouter Nicolas Cherel\, Doctorant à Télécom Paris (Institut Polytechnique de Paris).\nIl donnera un séminaire IMAGE\, le jeudi 2 février 2023 à 14h00\, en salle de séminaire F-200.\nTitre: « Patch and attention for image editing »\nRésumé : We show through two different examples that patch-based methods remain relevant despite the widespread use of neural networks for many image editing tasks.\nWe first present a patch-based algorithm for single image generation that performs as well as a neural network without requiring a costly training phase. We ensure visual fidelity and diversity of the results by carefully choosing the initialization of the algorithm.\nIn the second part\, we show that patch-based algorithms can benefit to modern techniques such as attention mechanisms. The use of attention has helped deep learning introduce long range dependencies but computing the full attention matrix is an expensive step with heavy memory and computational loads. We propose an efficient attention layer based on the stochastic algorithm PatchMatch\, which is used for determining approximate nearest neighbors. Our layer has a greatly reduced memory complexity compared to other attention layers\, scaling to high resolution images. \n  \nVenez nombreux!
URL:https://www.greyc.fr/event/seminaire-image-patch-and-attention-for-image-editing-nicolas-cherel/
LOCATION:ENSICAEN – Batiment F – Salle F-200\, 6 Bd Maréchal Juin\, Caen\, 14050\, France
CATEGORIES:General,Image,Seminaire Image
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BEGIN:VEVENT
DTSTART;VALUE=DATE:20230207
DTEND;VALUE=DATE:20230208
DTSTAMP:20260629T102025
CREATED:20230125T163951Z
LAST-MODIFIED:20230125T163951Z
UID:11078-1675728000-1675814399@www.greyc.fr
SUMMARY:Journée de la fédération NormaSTIC
DESCRIPTION:Venez profiter d’un programme qui inclut des exposés d’invités reconnus et des exposés relatifs à nos activités en Normandie et liées à la vie de la fédération. Parmi les exposés invités\, nous avons une décodeuse du numérique et une médaille de bronze du CNRS. \nLa journée aura lieu au Wip (à Colombelles) https://www.le-wip.com/ \nVoir le programme à https://www.normastic.fr/event/journee-de-la-federation-2023/ \n  \n 
URL:https://www.greyc.fr/event/journee-de-la-federation-normastic/
LOCATION:WIP\, Colombelles\, Rue des Ateliers 14460\,\, Colombelles\, 14460\, France
CATEGORIES:General
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230221T100000
DTEND;TZID=Europe/Paris:20230221T110000
DTSTAMP:20260629T102025
CREATED:20230125T164357Z
LAST-MODIFIED:20230213T105441Z
UID:11083-1676973600-1676977200@www.greyc.fr
SUMMARY:Séminaire Algorithmique : Mehdi Naima « Extending Brandes algorithm to improve betweenness centrality computation in temporal graphs with discrete and continuous time »
DESCRIPTION:Betweenness centrality has been a long subject of study in network science since it was introduced by Freeman in 1977. This centrality measure assesses the importance of nodes in a graph\, it has been used for example in social\, biological and research collaboration networks. Moreover\, betweenness centrality has been used in graph partitioning and community detection in the well-known Girvan-Newan algorithm.\n\nThis centrality measure is based on the enumeration of shortest paths passing through a node. A simple approach to compute betweenness centrality for all the nodes of a static graph is to use Floyd-Warshall algorithm that runs in O(n3). Brandes in 2001 published an algorithm that runs in O(nm + n2 log n) on weighted graphs\,  it is still considered one of the best theoretical results on the question.\n\nBetweenness centrality has also been extended to temporal graphs. Temporal graphs have edges that bear labels according to the time of the interactions between the nodes. Betweenness centrality has been extended to the temporal graph settings\, and the notion of paths has been extended to temporal paths. We will see that we are able to deploy Brandes algorithm to its full extent and improve the running time of recent results to O(nmT + n2Tlog(nT)). We will also discuss how Brandes algorithm can also be generalized to stream graphs which are dynamic graphs with continuous time and dynamicity on the nodes.
URL:https://www.greyc.fr/event/seminaire-algorithmique-mehdi-naima/
LOCATION:Sciences 3- S3 351
CATEGORIES:Amacc,General,Séminaire Algo
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230228T100000
DTEND;TZID=Europe/Paris:20230228T120000
DTSTAMP:20260629T102025
CREATED:20230125T165233Z
LAST-MODIFIED:20230224T142635Z
UID:11085-1677578400-1677585600@www.greyc.fr
SUMMARY:Séminaire Algorithmique : « Algorithmes pour la Dimension Métrique dans les graphes dirigés » Antoine Dailly (LIMOS\, Univ. Clermont-Ferrand)
DESCRIPTION:Résumé :\nLe problème de la Dimension Métrique d’un graphe se pose de la façon suivante : on cherche un ensemble R de sommets de taille minimale tel que\, pour toute paire de sommets du graphe\, il existe un sommet de R dont les distances aux deux sommets de la paire sont distinctes. Ce problème a été principalement étudié dans les graphes non-dirigés\, et il a attiré l’attention ces dernières années\, notamment en raison de sa difficulté : il est NP-complet et son meilleur facteur d’approximation en temps polynomial est log(n)\, y compris sur des classes de graphes très restreintes. \nNous considérons ce problèmes dans les graphes dirigés et orientés (la différence est que les graphes dirigés peuvent contenir des 2-cycles\, au contraire des graphes orientés)\, pour lesquels la Dimension Métrique a été récemment étudiée. Nous montrons que le problème reste NP-dur\, même dans les graphes orientés planaires bipartis de degré maximum 6. Cependant\, nous donnons des algorithmes linéaires résolvant le problème sur les arbres dirigés (les graphes dirigés dont le graphe sous-jacent est un arbre) et les orientations de graphes unicycliques. Enfin\, nous avons un algorithme paramétré par la largeur modulaire. \n\nAbstract:\nIn graph theory\, the Metric Dimension problem is the following: we are looking for a minimum-size set R of vertices\, such that for any pair of vertices of the graph\, there is a vertex from R whose two distances to the vertices of the pair are distinct. This problem has mainly been studied on undirected graphs\, and has gained a lot of attention in recent years\, mainly because of its difficulty: it is NP-complete and has a best polynomial-time approximation factor of log(n) even on very restricted graph classes. \nWe focus our study on directed and oriented graphs (the difference is that directed graphs may contain 2-cycles\, unlike oriented graphs)\, for which the Metric Dimension has been recently studied. We prove that Metric Dimension remains NP-hard\, even on planar bipartite oriented graphs of maximum degree 6. However\, we find linear-time algorithms solving the problem on directed trees (directed graphs whose underlying graph is a tree) and on orientations of unicyclic graphs. Finally\, we give a fixed-parameter-tractable algorithm for directed graphs when parameterised by modular-width.
URL:https://www.greyc.fr/event/seminaire-algorithmique-antoine-dailly-limos-univ-clermont-ferrand/
LOCATION:Sciences 3- S3 351
CATEGORIES:Amacc,General,Séminaire Algo
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