Links' Seminars and Public Events | ![]() ![]() |
Fri, April 30, 2021 10:00 am 12:00 pm | Présentation de NetworkDisk Je présenterais mon projet avec Bruno: NetworkDisk. Abstract and Title: TBA link to the project: TBA |
Fri, April 9, 2021 10:00 am 12:00 pm | Seminaire Pascal Weil titre: Problèmes algorithmiques en théorie des groupes infinis resumé: Malgré le titre très général, il s'agira uniquement de problèmes concernant les sous-groupes de groupes infinis, et même juste les sous-groupes de groupes libres. Les résultats et méthodes que je présenterai sont issus de près de 40 ans de littérature et sont dûs à un grand nombre d'auteurs. Je commencerai par poser le paysage, y compris pour ceux qui ne savent plus ce qu'est le groupe libre -- où l'on verra qu'on est, du point de vue algorithmique, dans une variante de la combinatoire des mots. Je présenterai ensuite l'outil central de la plupart des algorithmes efficaces sur les sous-groupes du groupe libre : la représentation de chaque sous-groupe finiment engendré par un graphe étiqueté et enraciné (disons : d'un automate :-)…) unique et facilement calculable à partir d'un ensemble de générateurs du sous-groupe considéré, qu'on appelle le graphe de Stallings. Le jeu consiste ensuite à traduire les problèmes algorithmiques sur les sous-groupes en problèmes algorithmiques sur les graphes de Stallings, et à résoudre ces problèmes de la façon la plus efficace possible. On considèrera notamment les problèmes suivants -- bon, juste le début de cette longue liste. - Le problème du mot généralisé : étant donnés k+1 éléments du groupe libre (ce sont des mots), le dernier appartient-il au sous-groupe engendré par les k premiers ? - Le problème de l'indice : étant donné un tuple d'éléments du groupe libre, le sous-groupe qu'ils engendrent est-il d'indice fini ? - Le problème de la base : étant donné un tuple d'éléments du groupe libre, trouver le rang, et une base du sous-groupe qu'ils engendrent. - Le problème de l'intersection : étant donnés deux tuples d'éléments du groupe libre, calculer l'intersection des sous-groupes qu'ils engendrent (ou calculer une base de cette intersection). - Le problème de la conjugaison : étant donnés deux tuples d'éléments du groupe libre, engendrent-ils le même sous-groupe ? deux sous-groupes conjugués ? - Et de nombreux autres problèmes (mots clés : minimalité de Whitehead, facteur libre, malnormalité, clôture par radical, clôture au sens de la topologie pro-p, etc…) title: Algorithmic problems in the theory of infinite groups abstract: In spite of the very general title, we will talk only about problems on subgroups of infinite groups, and in fact, only on subgroups of free groups . The results and methods I will present have been obtained over the past 40 years and are due to many researchers. I will start by setting the landscape, including for those who forgot what the free group is --- and we will see that we are dealing here, from the algorithmic point of view, with a variant of combinatorics on words. I will then present the tool that is central to most efficient algorithms on subgroups of free groups: the representation of each finitely generated subgroup by a labeled rooted graph (shall we say… an automaton?) which is unique and easily computable when a tuple of generators of the subgroup under consideration is given. This graph is called the Stallings graph. The game consists, then, in translating algorithmic problems on subgroups into algorithmic problems on Stallings graphs, and in solving these problems as efficiently as possible. We will discuss in particular the following problems (clearly: just the beginning of this long list). - The generalized word problem: given k+1 elements of the free group (these are words), does the last one belong to the subgroup generated by the k first ones? - The index problem: given a tuple of elements of the free group, does the subgroup they generate have finite index? - The basis problem: given a tuple of elements of the free group, find the rank and a basis of the subgroup they generate. - The intersection problem: given two tuples of elements of the free group, compute the intersection of the subgroups they generate (compute a basis of this intersection). - The conjugacy problem: given two tuples of elements of the free group, are the subgroups they generate equal? conjugated? - And many other problems (keywords: Whitehead minimality, free factors, malnormality, closure under radicals, closure in the sense of the pro-p topology, etc…) |
Fri, March 26, 2021 10:00 am 11:00 am | Séminaire Anne Etien Title: Managing structural and behavioral evolution in relational database: Application of Software Engineering techniques. Abstract: Relational databases play a central role in many information systems. Their schemas usually contain structural and behavioral entity descriptions. However, as any piece of software, they must continuously evolve to adapt to new requirements of a world in constant change. From an evolution point of view, problems are twofold: (1) relational database management systems do not allow inconsistencies i.e., no entity can reference a non existing entity; (2) stored procedures bodies are not described by meta-data i.e., DBMS as PostgreSQL consider stored procedure bodies as plain text and references to entities are unknown. As a consequence, evaluating the impact of an evolution of the database schema is a difficult task. In this seminar, we present a semi-automatic approach based on recommendations (sort of nested code transformations). Recommendations are proposed to architects who select the ones fitting their needs. Selected recommendations are then analysed and compiled to generate SQL script respecting the constraints imposed by the RDBMS. To support recommendations, we designed a meta-model for relational databases easing computation of change impact. We performed an experiment to validate the approach by reproducing a real evolution on a database. The results of our experiment show that our approach is able to reproduce exactly a manual modification in 75% less time. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri, March 19, 2021 10:00 am 12:00 pm | Seminar Pablo Ferragin Title: Theory and practice of learning-based compressed data structures Presenter: Giorgio Vinciguerra Abstract: We revisit two fundamental and ubiquitous problems in data structure design: predecessor search and rank/select primitives. We show that real data present a peculiar kind of regularity based on geometric considerations. We name it “approximate linearity”. We thus expand the horizon of compressed data structures by presenting two solutions for the problems above that discover, or “learn”, in a principled algorithmic way, these approximate linearities. We provide a walkthrough of these new theoretical achievements, also with a focus on open-source libraries and their experimental improvements. We conclude by discussing the plethora of research opportunities that these new learning-based approaches to data structure design open up. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri, March 12, 2021 10:00 am 12:00 pm | Seminar: Antonio AL SERHALI Title: Can Earliest Query Answering on Nested Streams be achieved in Combined Linear Time? |
Fri, February 19, 2021 10:00 am 11:00 am | Seminar: Bernardo Subercaseau Title: Foundations of Languages for Interpretability. Abstract: The area of interpretability in Machine Learning aims for the design of algorithms that we humans can understand and trust. One of the fundamental questions of interpretability is: given a classifier M, and an input vector x, why did M classify x as M(x)? In order to approximate an answer to this "why" question, many concrete queries, metrics and scores have emerged as proxies, and their complexity has been studied over different classes of models. Many of these analyses are ad-hoc, but they tend to agree on the fact that these queries and scores are hard to compute over Neural Networks, but easy to compute over Decision Trees. It is thus natural to think of a more general approach, like a query language in which users could write an arbitrary number of different queries, and that would allow for a generalized study of the complexity of interpreting different ML models. Our work proposes foundations for such a language, tying to First Order Logic, as a way to have a clear understanding of its expressiveness and complexity. We manage to define a minimalistic structure over FO that allows expressing many natural interpretability queries over models, and we show that evaluating such queries can be done efficiently for Decision Trees, in data-complexity. Zoom link: univ-lille-fr.zoom.us/j/95419000064 |
Fri, February 12, 2021 10:00 am 12:00 pm | Seminar: Florent Capelli Title: Regularizing the delay of enumeration algorithms Zoom link: univ-lille-fr.zoom.us/j/95419000064 Abstract: Enumeration algorithms are algorithms whose goal is to output the set of all solutions to a given problem. There exists different measures for the quality of such algorithm, whose relevance depends on what the user wants to do with the solutions set. If the goal of the user is to explore some solutions or to transform the solutions as they are outputted with a stream-like algorithm, a relevant measure of the complexity of an enumeration algorithm is the delay between the output of two distinct solutions. Following this line of thoughts, significant efforts have been made by the community to design polynomial delay algorithms, that is, algorithms whose delay between the output of two new solutions is polynomial in the size of the input. While this measure is interesting, it is not always completely necessary to have a bound on the delay and it is enough to ask for a guarantee that running the algorithm for O(t poly(n)) will result in the output of at least t solutions. Of course, by storing each solution seen and outputting them regularly, one can simulate a polynomial delay but if the number of solutions is large, it may result in a blow up in the space used by the enumerator. In this talk, we will present a new technique that allow to transform such algorithm into polynomial delay algorithm using polynomial space. This is joint work with Yann Strozecki. |