Links' Seminars and Public Events | ![]() ![]() |
Fri, December 10, 2021 11:00 am 12:00 pm | Séminaire Sebastien Tavenas Title: Bornes inférieures superpolynomiales pour les circuits de profondeur constante Abstract: Tout polynôme multivarié P(X_1,...,X_n) peut être écrit comme une somme de monômes, i.e., une somme de produits de variables et de constantes du corps. La taille naturelle d'une telle expression est le nombre de monômes. Mais, que se passe-t-il si on rajoute un nouveau niveau de complexité en considérant les expressions de la forme : somme de produits de sommes (de variables et de constantes) ? Maintenant, il devient moins clair comment montrer qu'un polynôme donné n'a pas de petite expression. Dans cet exposé nous résoudrons exactement ce problème. Plus précisément, nous prouvons que certains polynômes explicites n'ont pas de représentations "somme de produits de sommes'' (SPS) de taille polynomiale. Nous pouvons aussi obtenir des résultats similaires pour les SPSP, SPSPS, etc... pour toutes les expressions de profondeur constante. " |
Thu, November 25, 2021 2:00 pm 3:00 pm | Nofar Carmeli in Links' Seminar |
Fri, October 29, 2021 11:00 am 12:00 pm | Séminaire Antoine Amarilli |
Fri, October 22, 2021 11:00 am 12:00 pm | Mikaël Monet in Links' Seminar |
Fri, October 15, 2021 11:00 am 12:00 pm | Claire Soyez-Martin in Links' seminar |
Fri, September 17, 2021 11:00 am 12:00 pm | Séminaire Corentin Barloy Title: Stackless Processing of Streamed Trees Abstract: Processing tree-structured data in the streaming model is a chal-lenge: capturing regular properties of streamed trees by means of astack is costly in memory, but falling back to finite-state automata drastically limits the computational power. We propose an intermediate stackless model based on register automata equipped with a single counter, used to maintain the current depth in the tree. We explore the power of this model to validate and query streamed trees. Our main result is an effective characterization of regular path queries (RPQs) that can be evaluated stacklessly—with and without registers. In particular, we confirm the conjectured characterization of tree languages defined by DTDs that are recognizable without registers, by Segoufin and Vianu (2002), in the special case of tree languages defined by means of an RPQ. Link: paperman.name/data/pub.....0.pdf lille-Salle |
Fri, September 10, 2021 10:00 am 11:00 am | Séminaire de Patrick Baillot titre: Type-based complexity analysis in a parallel process calculus Abstract: Some type systems have been designed to analyse statically the time coplexity of functional languages. A natural question is whether this approach can be extended to parallel languages. We address this problem for the Pi-calculus, a paradigmatic calculus for parallel and concurrent computation. In Pi-calculus, processes communicate through channels that can carry values and channel names. We will define notions of sequential and parallel complexity for Pi-calculus, and present a type system that provides an upper bound on the time complexity of processes. This is based on joint work with Alexis Ghyselen (ESOP 2021). Based on: link.springer.com/chap.....9-3_3 |
Fri, July 9, 2021 all day | Seminar - Antonio AL SERHALI Title: Integrating Schema-Based Cleaning into Automata Determinization Abstract : Schema-based cleaning for automata on trees or nested words was proposed recently to compute smaller deterministic automata for regular path queries on data trees. The idea is to remove all rules and states, from an automaton for the query, that are not needed to recognize any tree recognized by a given schema automaton. Unfortunately, how- ever, deterministic automata for nested words may still grow large for au- tomata for XPath queries, so that the much smaller schema-cleaned ver- sion cannot always be computed in practice. We therefore propose a new schema-based determinization algorithm that integrates schema-based cleaning directly. We prove that schema-based determinization always produces the same deterministic automaton as schema-based cleaning after standard determinization. Nevertheless, the worst-case complex- ity is considerably lower for schema-based determinization. Experiments confirm the relevance of this result in practice. |
Fri, June 4, 2021 10:00 am 12:30 pm | Séminaire Pierre Ohlmann Zoom link: univ-lille-fr.zoom.us/j/95419000064 Titre: Lower bound for arithmetic circuits via the Hankel matrix Abstract: We study the complexity of representing polynomials by arithmetic circuits in both the commutative and the non-commutative settings. To analyse circuits we count their number of parse trees, which describe the non-associative computations realised by the circuit. In the non-commutative setting a circuit computing a polynomial of degree d has at most 2^{O(d)} parse trees. Previous superpolynomial lower bounds were known for circuits with up to 2^{d^{1/3-ε}} parse trees, for any ε>0. Our main result is to reduce the gap by showing a superpolynomial lower bound for circuits with just a small defect in the exponent for the total number of parse trees, that is 2^{d^{1-ε}}, for any ε>0. In the commutative setting a circuit computing a polynomial of degree d has at most 2^{O(d \\log d)} parse trees. We show a superpolynomial lower bound for circuits with up to 2^{d^{1/3-ε}} parse trees, for any ε>0. When d is polylogarithmic in n, we push this further to up to 2^{d^{1-ε}} parse trees. While these two main results hold in the associative setting, our approach goes through a precise understanding of the more restricted setting where multiplication is not associative, meaning that we distinguish the polynomials (xy)z and yz). Our first and main conceptual result is a characterization result: we show that the size of the smallest circuit computing a given non-associative polynomial is exactly the rank of a matrix constructed from the polynomial and called the Hankel matrix. This result applies to the class of all circuits in both commutative and non-commutative settings, and can be seen as an extension of the seminal result of Nisan giving a similar characterization for non-commutative algebraic branching programs. Our key technical contribution is to provide generic lower bound theorems based on analyzing and decomposing the Hankel matrix, from which we derive the results mentioned above. The study of the Hankel matrix also provides a unifying approach for proving lower bounds for polynomials in the (classical) associative setting. We demonstrate this by giving alternative proofs of recent lower bounds as corollaries of our generic lower bound results. |
Fri, May 28, 2021 10:00 am 11:00 am | Seminar Anastasia Dimou Title: Knowledge graph generation and validation |
Fri, May 21, 2021 10:00 am 12:00 pm | Seminar Dimitrios Myrisiotis Title : One-Tape Turing Machine and Branching Program Lower Bounds for MCSP Abstract: eccc.weizmann.ac.il/report/2020/103/ Speaker' webpage : dimyrisiotis.github.io/ zoom |
Fri, May 7, 2021 10:00 am 12:00 pm | Seminar Nicole Schweikardt Title: Spanner Evaluation over SLP-Compressed Documents Abstract: We consider the problem of evaluating regular spanners over compressed documents, i.e., we wish to solve evaluation tasks directly on the compressed data, without decompression. As compressed forms of the documents we use straight-line programs (SLPs) -- a lossless compression scheme for textual data widely used in different areas of theoretical computer science and particularly well-suited for algorithmics on compressed data. In terms of data complexity, our results are as follows. For a regular spanner M and an SLP S that represents a document D, we can solve the tasks of model checking and of checking non-emptiness in time O(size(S)). Computing the set M(D) of all span-tuples extracted from D can be done in time O(size(S) size(M(D))), and enumeration of M(D) can be done with linear preprocessing O(size(S)) and a delay of O(depth(S)), where depth(S) is the depth of S's derivation tree. Note that size(S) can be exponentially smaller than the document's size |D|; and, due to known balancing results for SLPs, we can always assume that depth(S) = O(log(|D|)) independent of D's compressibility. Hence, our enumeration algorithm has a delay logarithmic in the size of the non- compressed data and a preprocessing time that is at best (i.e., in the case of highly compressible documents) also logarithmic, but at worst still linear. Therefore, in a big-data perspective, our enumeration algorithm for SLP-compressed documents may nevertheless beat the known linear preprocessing and constant delay algorithms for non-compressed documents. [This is joint work with Markus Schmid, to be presented at PODS'21.] Link to the paper: arxiv.org/pdf/2101.10890.pdf for the paper at least Link to the ACM video: TBA |
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. |
Fri, January 15, 2021 10:00 am 12:00 pm | Séminaire de Kim Nguyễn Titile: The BOLDR project Abstract: I n this presentation, I will give an account of the BOLDR project and perspectives in the field of language integrated queries. Several classes of solutions allow programming languages to express queries: specific APIs such as JDBC, Object-Relational Mappings (ORMs) such as Hibernate, and language-integrated query frameworks such as Microsoft's LINQ. However, most of these solutions do not allow for efficient cross-databases queries, and none allow the use of complex application logic from the programming language in queries. We study the design of a new language-integrated query framework called BOLDR that allows the evaluation in databases of queries written in general-purpose programming languages containing application logic, and targeting several databases following different data models. In this framework, application queries are translated to an intermediate representation. Then, they are typed with a type system extensible by databases in order to detect which database language each subexpression should be translated to. This type system also allows us to detect a class of errors before execution. Next, they are rewritten in order to avoid query avalanches and make the most out of database optimizations. Finally, queries are sent for evaluation to the corresponding databases and the results are converted back to the application. Our experiments show that the techniques we implemented are applicable to real-world database applications, successfully handling a variety of language-integrated queries with good performances. This talk will give an overview of what has been achieved so far (mainly in the context of Julien Lopez' PhD Thesis) and will glimpse at preliminary work that is being done in the context of a collaboration with Oracle Labs. |
Fri, January 8, 2021 10:45 am 12:30 pm | Séminaire @ Lê Thành Dũng (Tito) Nguyễn Title: The planar geometry of first-order string transductions (joint work with Pierre Pradic) Abstract: hal.archives-ouvertes......ument We propose a new machine model recognizing star-free languages, with a geometric flavor. Our starting point is the characterization of regular languages using two-way automata (2DFA). The idea is to take seriously the visual representations found throughout the literature of the behavior of a 2DFA on a word ; by putting a total order on the set of states, one can formally define what it means for such a behavior to be planar, in a sense analogous to the planarity of combinatorial maps. Star-free languages are then exactly the languages recognized by "planar 2DFA". We also show that the corresponding planar transducer model characterizes the class of first-order transductions (a.k.a. aperiodic regular functions). If time allows, the talk will briefly discuss the connections of this work with the non-commutative lambda-calculus (cf. our recent paper Aperiodicity in a non-commutative logic, ICALP'20). |
Thu, December 17, 2020 2:00 pm 4:00 pm | Nofar Carmeli Speaker: Nofar Carmeli (nofar.carme.li/) Zoom link: univ-lille-fr.zoom.us/j/95419000064 Title: The Complexity of Answering Unions of Conjunctive Queries. Abstract: We discuss the fine-grained complexity of enumerating the answers to a query over a relational database. With the ideal guarantees, linear time is required before the first answer to read the input and determine its existence, and then we need to print the answers one by one. Consequently, we wish to identify the queries that can be solved with linear preprocessing time and constant or logarithmic delay between answers. A known dichotomy classifies CQs into those that admit such enumeration and those that do not. The computationally expensive component of query answering is joining tables, which can be done efficiently if and only if the join query is acyclic. However, the join query usually does not appear in a vacuum; for example, it may be part of a larger query, or it may be applied to a database with dependencies. We inspect how the complexity changes in these settings and chart the borders of tractability within. In addition, we consider the task of enumerating query answers with a uniformly random order, and we propose to do so using an efficient random-access structure for representing the set of answers. We also prove conditional lower bounds showing that our algorithms capture all tractable queries in some cases. Among our results, we show that a union of tractable conjunctive queries may be intractable w.r.t. random access; on the other hand, a union of intractable conjunctive queries may be tractable w.r.t. enumeration. |
Fri, December 11, 2020 10:00 am 11:30 am | Alexandre Vigny Title: Elimination Distance to Bounded Degree on Planar Graphs Link to the zoominar: univ-lille-fr.zoom.us/j/95419000064 Abstract: What does it mean for a graph to almost be planar? Or to almost have bounded degree? On such simple graphs classes, some difficult algorithmic problems become tractable. Ideally, one would like to use (or adapt) existing algorithms for graphs that are "almost" in such a simple class. In this talk, I will discuss the notion of elimination distance to a class C, a notion introduced by Bulian and Dawar (2016). The goals of the talk are: 1) Define this notion, and discuss why it is relevant by presenting some existing results. 2) Show that we can compute the elimination distance of a given planar graph to the class of graph of degree at most d. I.e. answer the question: "Is this graph close to a graph of bounded degree?" The second part is the result of a collaboration with Alexandre Lindermayer and Sebastian Siebertz. |