Seminars

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.

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).

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.

Title: Stackless processing of streamed trees

Abstract: In this talk, I will first present the state of the art of efficiency implementation of streaming-text algorithms on modern architecture. Then some recent results on the extraction of information on streamed of structured documents without stack overhead.

For more info: paperman.name/data/pub.....d.pdf

Title: ShEx Learning from Typed Graphs

Abstract: In knowledge graphs, schemas are becoming a new asset to describe the organization of data. The new world-leading format Shex is becoming a de-facto standard in the industry that allows defining flexible and powerful schemas.

In this context, the inference of schemas can become a solution to provide shex expressions that describe already existing data. Typically, the inference starts from untyped graphs. However, these tasks appears to be more complex than expected in general, and is possible only for subclasses of Shex.

The inference of schemas from typed graph gives a baseline for those algorithms. Its comprehension allows to understand better the underlying difficulties of the task. It presents already unexpected difficulties.

We present an algorithm that infers Shex-defined schemas from fully typed graphs. We also present some encountered difficulties, as well as the limitations of the approach.

Introduction to argumentation theory

Title: General Dynamic Yannakakis: Conjunctive Queries with Theta Joins Under Updates
Abstract:
The ability to efficiently analyze changing data is a key requirement
of many real-time analytics applications like Stream Processing,
Complex Event Recognition, Business Intelligence, and Machine
Learning.

Traditional approaches to this problem are based either on the
materialization of subresults (to avoid their recomputation) or on the
recomputation of subresults (to avoid the space overhead of
materialization). Both techniques have recently been shown suboptimal:
instead of fully materializing results and subresults, one can
maintain a data structure that supports efficient maintenance under
updates and can quickly enumerate the full query output, as well as
the changes produced under single updates.

In our work we are concerned with designing a practical family of
algorithms for dynamic query evaluation based on this idea, and for
queries featuring both equi-joins and inequality joins, as well as
certain forms of aggregation. Our main insight is that, for acyclic
conjunctive queries, such algorithms can naturally be obtained by
modifying Yannakakis' seminal algorithm for processing acyclic joins
in the static setting.

In this talk I present the main ideas behind this modfication,
offset it against the traditional ways of doing incremental view
maintenance, and discuss recent extensions such as dealing with
general theta-joins.

Elles présenteront un outil qui prend en entrée une requête SQL et sa compilation par Postrgres sous forme de plan d'exécution, et démontre (avec Coq) que la requête initiale est équivalente au plan d'exécution.

Title: Regular Matching and Inclusion on Compressed Tree Patterns with Context Variables

Abstract: We study the complexity of regular matching and inclusion for compressed tree patterns extended by context variables. The addition of context variables to tree patterns permits us to properly capture compressed string patterns but also compressed patterns for unranked trees with tree and hedge variables. Regular inclusion for the latter is relevant to certain query answering on Xml streams with references.

Title: Containment for Probabilistic automata.

Abstract: This is an ICALP 2018 paper. We analyze when the model of probabilistic
automata has decidable properties, when restricting the ambiguity. The
notion of ambiguity is usually used in weighted automata or transducers,
but we follow a recent paper by Fijalkow, Riveros and Worrell, which
introduced this approach. We do not solve everything but our decidability
results rely unexpectedly on Schanuel's conjecture and we provide some
geometric intuition behind the hardness of the problem.

Abstract: The time complexity of 1-limited automata is investigated from a
descriptional complexity view point. Though the model recognizes
regular languages only, it may use quadratic time in the input length.
We show that, with a polynomial increase in size and preserving
determinism, each 1-limited automaton can be transformed into a
linear-time equivalent one. We also obtain polynomial transformations
into related models, including weight-reducing Hennie machines (i.e.,
one-tape Turing machines syntactically forced to operate in
linear-time), and we show exponential gaps for converse
transformations in the deterministic case.

Title: Combined Complexity of Probabilistic Query Evaluation

Abstract:
Query evaluation over probabilistic databases (probabilistic query evaluation, or PQE) is known to be intractable in many cases, even in data complexity, i.e., when the query is fixed. Although some restrictions of the queries and instances have been proposed to lower the complexity, these known tractable cases usually do not apply to combined complexity, i.e., when the query is not fixed. This talk gives an overview of my PhD research, which investigates which queries and instances ensure the tractability of PQE in combined complexity.

I will first present our work on PQE of conjunctive queries on binary signatures, which can be rephrased as a probabilistic graph homomorphism problem. We restrict the query and instance graphs to be trees and show the impact on the combined complexity of diverse features such as edge labels, branching, or connectedness. This is joint work with Antoine Amarilli and Pierre Senellart and was presented at PODS'2017.

Second, we will explore the combined complexity of evaluating queries on treelike databases, i.e., databases whose treewidth is bounded by a constant. We introduce a class of queries (named 'CFG-Datalog') which generalizes many known query languages that are tractable in this context. Specifically, we show that the (non-probabilistic) evaluation of CFG-Datalog on treelike databases can be solved with complexity linear in the product of the instance size and of the query size. In the process, we introduce a new representation of the provenance of a query on a database, based on cyclic Boolean circuits. This is joint work with Antoine Amarilli, Pierre Bourhis, and Pierre Senellart, and was presented at ICDT'2017.

Last, we will move to the field of knowledge compilation and present our work that connects various notions of width for Boolean circuits. We show that circuits of bounded treewidth can be efficiently compiled into structured deterministic decomposable normal forms (d-SDNNFs), which in particular allows efficient probability computation. We show the implications of this result for PQE of CFG-Datalog on treelike databases. We also prove general lower bounds on knowledge compilation formalisms, which imply lower bounds for provenance computation. This is joint work with Antoine Amarilli and Pierre Senellart and was presented at ICDT'2018.

This talk connects two classical areas of theoretical computer science: descriptive complexity and distributed computing. The former is a branch of computational complexity theory that characterizes complexity classes in terms of equivalent logical formalisms. The latter studies algorithms that run in networks of interconnected processors.

Although an active field of research since the late 1970s, distributed computing is still lacking the analogue of a complexity theory. One reason for this may be the large number of distinct models of distributed computation, which make it rather difficult to develop a unified formal framework. In my talk, I will outline how the descriptive approach, i.e., connections to logic, could be helpful in this regard.

Paul présentera le papier de Sylvain, Aurélien et Paul, soumis à LICS 2018, sur le sujet des transducteurs d'arbres d'ordre supérieur.

In this talk I present a logic, called LT, to express properties of transductions, i.e. binary relations from input to output (finite) words. I argue that LT is a suitable candidate as a specification language for verification of non reactive systems, extending the successful approach of verifying synchronous systems via Mealy Machines and MSO.

In LT, the input/output dependencies are modelled via an origin function which associates to any position of the output word, the input position from which it originates. LT is well-suited to express relations (which are not necessarily functional), and can express all regular functional transductions, i.e. transductions definable for instance by deterministic two-way transducers.
Despite its high expressive power, LT has decidable satisfiability problems. The main contribution is a synthesis result: it is always possible to synthesis a regular function which satisfies the specification.

Finally, I explicit a correspondence between transductions and data words. As a side-result, we obtain a new decidable logic for data words.

Cypher is a query-language for property-graphs. It was originally designed and implemented as part of the Neo4j graph database, and it is currently used by several commercial database products and researchers. The semantics of Cypher queries is currently described using natural language and, as a result, it is often not well defined. This work is part of a project to define a full denotational semantics of Cypher queries. The talk will first present the main features of Cypher through examples, including the core mecanism: graph pattern-matching, and then will describe the formal semantics in its current state.

Cours extérieur de Gérard Berry du Collège de France

Le centre Inria Lille - Nord Europe reçoit Gérard Berry, du Collège de France pour son cours sur la photographie numérique. Ce cours se prolongera par un séminaire de Stéphane Huot, responsable de l'équipe Mjolnir. Cette manifestation sera suivie d'un cocktail au cours duquel Isabelle Herlin, directrice du centre de recherche Inria Lille - Nord Europe présentera ses voeux.

Date : 31/01/2018
Lieu : Lilliad, Campus Université Lille - sciences et technologies - 2 avenue Jean Perrin, Villeneuve d'Ascq

Programme

15h30 : Accueil

15h45 : Introduction par Isabelle Herlin

16h - 17h30 : Cours de Gérard Berry, "La photographie numérique, un parfait exemple de la puissance de l'informatique"

17h30 - 18h30 : Séminaire de Stéphane Huot, "Interaction humain-machine : passé composé et futur simple... ou l'inverse"

18h30 - 18h45 : Questions aux deux orateurs

19h - 20h30 : Cocktail

Cours de Gérard Berry

Bio express : Gérard Berry est informaticien, professeur au Collège de France où il est titulaire de la chaire d'Algorithmes, machines et langages.


Résumé

L’appareil photo numérique est un excellent exemple de l’évolution actuelle des systèmes cyberphysiques, c’est-à-dire des systèmes couplant intimement mécanique, physique, électronique et logiciel. C’est aussi un exemple merveilleux et accessible à tous de la puissance des méthodes de l’informatique par rapport à celles de la physique et de la mécanique seules. Le cours présentera la panoplie des algorithmes embarqués dans les appareils photos modernes et dans les logiciels de postproduction, puis discutera l’impact majeur qu’ils ont sur la conception des appareils et des objectifs, totalement bouleversée en ce moment, et celui qu’ils ont sur les photographes professionnels ou amateurs.

La photographie argentique, fort ancienne, n’a que lentement progressé au cours du XXe siècle : amélioration lente des pellicules et papiers, introduction de l’exposition automatique calculée analogiquement à partir de cellules photo-électriques, visée télémétrique ou reflex, tout cela a demandé des dizaines d’années. Au contraire, à partir de la commercialisation du premier appareil numérique en 1990, la photographie numérique a évolué extrêmement vite. En 2003, on trouvait déjà des appareils semi-professionnels corrects et, dès 2009, des appareils reflex de haute qualité à un prix abordable. Maintenant, il existe toute une panoplie d’appareils de tailles variées, tous capables de fournir des images de grande qualité. Même les téléphones sont devenus de très bons appareils photos et caméras vidéo, principalement grâce aux algorithmes qu’ils mettent en œuvre. Comme ils savent faire bien d’autres choses, par exemple envoyer immédiatement les images sur Internet, ils sont en train de remplacer les anciens petits appareils compacts et de servir d’équipement unique pour les photographes occasionnels et pour tous dans les pays où la photo argentique était d’un coût inabordable pour les habitants. La logique de la photo numérique est ainsi devenue très différente de celle de l’argentique, ce qui n’empêche cependant pas que cette dernière garde toujours les faveurs de certains artistes.

Qu’est-ce qui a permis cette révolution et pourquoi est-elle allée aussi vite ? Il y a trois raisons principales : la conception par les physiciens et la fabrication industrielle en grand volume de capteurs de haute qualité ; l’augmentation considérable de la puissance et la diminution de la dépense énergétique des ordinateurs embarqués, grâce à la fameuse loi de Moore ; enfin, et surtout, l’amélioration continue des algorithmes de la photographie, qui jouent en fait un rôle plus important que celui des capteurs. Dans les quinze dernières années, nous avons gagné au moins 4 degrés de sensibilité, dont les trois quarts grâce aux algorithmes. Même des appareils aux capteurs relativement petits savent faire des photos de très haute qualité à 3200 ISO, ce qui était complètement impossible avec l’argentique.

Le cours détaillera d’abord la suite des transformations algorithmiques subtiles qui permettent le développement des images des données numériques du capteur, aboutissant à l’image finale en gérant au mieux la lumière, la netteté et le bruit. Ensuite, il étudiera les algorithmes dédiés à la correction automatique des divers défauts optiques des objectifs ; il montrera que la puissance de ces algorithmes fait que ces objectifs ne seront plus conçus comme auparavant : leur conception intègre désormais totalement physique et algorithmique, fournissant des optiques de meilleure qualité, moins encombrantes, plus légères et moins chères. Il insistera sur l’importance que prennent les nouveaux traitements fondés sur la fusion de prises de vue successives pour l’amélioration de la qualité selon divers objectifs (lumière, bruit, profondeur de champ, etc.), en particulier pour les téléphones. Il montrera pourquoi fonder les nouveaux appareils directement sur les algorithmes modifie de plus en plus le cœur de leur conception, ce qui fait que bien d’autres nouveautés surprenantes pourront apparaître. Des évolutions similaires bouleversent d’ailleurs tout autant les imageries médicale et astronomique.

Enfin, le cours soulignera l’importance des nouveaux algorithmes destinés à l’amélioration de l’ergonomie de la prise de vue, qui rendent la vie technique du photographe bien plus facile sur quasiment tous les aspects : interaction humain-machine bien conçue, stabilisation du capteur et de l’objectif pour supprimer le flou de bougé, gestion sophistiquée de la lumière et de la mise au point, nombreuses aides à la prise de vue dans le viseur devenant électronique, liaison directe avec les ordinateurs et téléphones.
Séminaire de Stéphane Huot

Résumé

Bien avant l'avènement des ordinateurs personnels, de l'internet et des smartphones , l'Interaction Homme-Machine (IHM) était déjà une préoccupation au cœur de certaines des visions qui ont contribué à forger l'informatique moderne, qu'elle soit personnelle ou professionnelle. Pour autant, la conception et l'étude des interactions est encore souvent considérée comme secondaire dans la conception des systèmes, la priorité étant souvent mise sur le développement des fonctionnalités plutôt que sur les moyens pour les utiliser.

Cette situation s'est progressivement améliorée, avec notamment l'avènement des dispositifs tactiles (smartphones et tablettes) ou de divertissement (consoles de jeux) pour lesquels l'argument de simplicité d'utilisation a détrôné celui de la puissance intrinsèque. Cela a bien évidemment permis de populariser et démocratiser l'accès à la technologie. Mais une conséquence est, selon nous, un relatif appauvrissement des possibilités offertes par ces technologies paradoxalement plus puissantes que jamais. En masquant la complexité plutôt qu'en aidant à la maîtriser, en entretenant le mythe qu'avec ces dispositifs il est aisé pour chacun de faire beaucoup sans efforts, la tendance est à sacrifier le potentiel de l'outil informatique et la performance des utilisateurs pour la rapidité de prise en main, sans permettre un usage plus avancé, plus performant, et peut-être plus gratifiant pour l'utilisateur.

Cet équilibre entre simplicité d'usage et puissance de l'outil est un compromis difficile à trouver, et c'est selon nous un des défis et une difficulté majeure de l'IHM : observer et comprendre les phénomènes sensorimoteurs et psychomoteurs, cognitifs, sociaux et technologiques mis en œuvre lors de l'interaction entre des personnes et des systèmes afin d'améliorer cette interaction et d'en guider la conception pour encapaciter les utilisateurs. Le but étant finalement de leur permettre de réaliser ce qu'il leur serait impossible de faire sans cet outil, même si cela requiert de leur part des efforts certains d'apprentissage.
Dans ce séminaire, nous commencerons par présenter ce qu'est l'Interaction Homme-Machine en tant que domaine de recherche avec ses objectifs, ses méthodes et ses pratiques.

Ensuite, au travers d'une brève histoire de l'informatique sous le prisme de l'interaction, nous évoquerons quelques innovations d'aujourd'hui qui découlent des visions de pionniers du domaine, en considérant notamment ce compromis simplicité / puissance de l'outil. Nous verrons aussi avec des exemples et contre-exemples issus de nos environnements numériques actuels, ainsi qu'avec des travaux de recherche récents, que ces visions portent encore de nombreux défis présents et futurs de l'IHM. En particulier, nous conclurons en discutant de la nécessité d'adopter une approche centrée utilisateur et interaction à l'heure des grands défis scientifiques, technologiques et sociétaux du numérique tels que la conception des systèmes autonomes ou le traitement et l'exploitation automatique des données.

Mots-clés : Machines et langages Algorithme Interaction Homme machine (IHM).

Cet exposé se veut une introduction à la transformation de
programmes datalog. En particulier, je présenterai la transformation
appelée "supplementary magic set rewriting" qui permet d'obtenir des
programmes datalog dont l'évaluation semi-naïve se comporte de façon
similaire à l'évaluation des programmes originaux par résolution SLD. Je
montrerai l'algorithme et des exécutions de programmes sur des exemples
issus de problèmes d'analyses grammaticales.

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.

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.

Counter-Example-Guided Abstraction Refinement (CEGAR) has been very successful in model checking.
Since then, it has been applied to many different problems. It is especially proved to be a highly successful practical approach for solving the PSPACE complete QBF problem. In this paper, we propose a new CEGAR-like approach for tackling PSPACE complete problems that we call RECAR (Recursive Explore and Check Abstraction Refinement). We show that this generic approach is sound and complete. Then we propose a specific implementation of the RECAR approach to solve the modal logic K satisfiability problem. We implemented both CEGAR and RECAR approaches for the modal logic K satisfiability problem within the solver MoSaiC. We compared experimentally those approaches to the state-of-the-art solvers for that problem. The RECAR approach outperforms the CEGAR one for that problem and also compares favorably against the state-of-the-art on the benchmarks considered.

In database management, one of the principal task is to optimize the queries to evaluate them efficiently. It is in particular the case for recursive queries for which their evaluation can lead to crawl all the database. In particular, one of the main question is to minimize the queries in order to avoid to evaluate useless parts of the query. The core theoretical question around this line of work is the problem of inclusion of a query in another. Interestedly, this question is related to an important question in IA which is to answer a query when the data is incomplete but rules are given to derive new information. This problem is called certain query answering. In both context, if both problem are undecidable in general, there are fragments based on guardedness that are decidable due to the fact there exists witness of the problems that have a bounded tree width and that their encoding in trees is regular. Furthermore, the queries can be translated in MSO. In both contexts, Courcelle’s Theorems imply the decidability of both problems. I will present to the different results on the translation of logic class of formula for our problems into tree automata to obtain tight bounds to the problems of inclusion of recursive queries or certain query answering.

Abstract:

We show that equivalence of deterministic top-down tree-to-string transducers is decidable,
thus solving a long standing open problem in formal language theory.
We also present efficient algorithms for subclasses:
polynomial time for total transducers with unary output alphabet (over a given top-down regular domain language),
and co-randomized polynomial time for linear transducers, these results are obtained using techniques from multi-linear algebra.
For our main result, we prove that equivalence can be certified by means of inductive invariants using polynomial ideals.
This allows us to construct two semi-algorithms, one searching for a proof of equivalence, one for a witness of non-equivalence.

Abstract: We consider the problem of computing a conjunctive query on a large database in a parallel setting with p servers. Unlike traditional query processing, the complexity is no longer dominated by the number of disk accesses. Typically, a query is evaluated by a sufficiently large number of servers such that the entire data can be kept in the main memory of these servers. The dominant cost becomes that of communicating data and synchronizing among the servers.
I will present some interesting results in [1, 2, 3, 4] dealing with the communication complexity of massively parallel computation of a query. The computation is performed in "rounds".
First, I will present the Massively Parallel Communication (MPC) model to analyze the tradeoff between the number of rounds and the amount of communication required in a massively parallel computing environment.
Then I will present the HyperCube (HC) algorithm that computes a full conjunctive query q in one round.
I will discuss the communication complexity [2]. The main result is the optimal load O(m/p1/τ ) where τ is the fractional vertex cover of the hypergraph of q and m the input data size.

References
[1] Parallel Evaluation of Conjunctive Queries. Paris Koutris, Dan Suciu PODS2011
[2] Communication Steps for Parallel Query Processing. Paul Beame, Paris Koutris, Dan Suciu PODS2013
[3] Skew in Parallel Query Processing. Paul Beame, Paris Koutris Dan Suciu PODS'2014
[4] Worst-Case Optimal Algorithms for Parallel Query Processing. Paris Koutris, Paul Beame, Dan Suciu ICDT2016

Abstract:
The challenge that we tackle in this thesis is the problem of how to answer
XPath queries on XML streams with low latency, full coverage, high time
efficiency, and low memory costs. We first propose to approximate earli-
est query answering for navigational XPath queries by compilation to early
nested word automata. It turns out that this leads to almost optimal la-
tency and memory consumption. Second, we contribute a formal semantics
of XPath 3.0. It is obtained by mapping XPath to the new query language
λXP that we introduce. We then show how to compile λXP queries to net-
works of early nested word automata, and develop streaming algorithms for
the latter. Thereby we obtain a streaming algorithm that indeed covers all of
XPath 3.0. Third, we develop an algorithm for projecting XML streams with
respect to the query defined by an early nested word automaton. Thereby
we are able to make our streaming algorithms highly time efficient. We have
implemented all our algorithms with the objective to obtain an industrially
applicable streaming tool. It turns out that our algorithms outperform all
previous approaches in time efficiency, coverage, and latency.

Abstract:
In this talk, I will present a connection between the streaming complexity and the circuit complexity of regular languages through a notion of streaming by block . This result provides tight constructions of boolean circuits computing an automaton, thanks to some classical and recent results on the circuit complexity of regular languages. I will apply this framework to the schema validation in streaming of XML-documents.

Abstract: Higher-order constructions make their way into main stream
programming languages like Java, C++, python, rust... These
constructions bring new challenges to the verification of programs as
they make their control flow more complex.

In this talk, I will present how methods coming from denotational
semantics can prove decidable the verification of certain properties of
higher-order programs. These properties are expressed by means of finite
state automata of the possibly infinite execution trees generated by the
programs and can capture safety properties but also liveness and
fairness properties.

Higher-order pushdown systems and ground tree rewriting systems can
be seen as extensions of suffix word rewriting systems. Both classes
generate infinite graphs with interesting logical
properties. Indeed, the satisfaction of any formula written in
monadic second order logic (respectively first order logic with
reachability predicates) can be decided on such a graph.

The purpose of this talk is to propose a common extension to both
higher-order stack operations and ground tree rewriting. We introduce a model
of higher-order ground tree rewriting over trees labelled by
higher-order stacks (henceforth called stack trees), which syntactically
coincides with ordinary ground tree rewriting at order 1 and with the dynamics
of higher-order pushdown automata over unary trees. The infinite
graphs generated by this class have a decidable first order logic with
reachability.

Formally, an order n stack tree is a tree labelled by order n-1 stacks.
Operations of ground stack tree rewriting are represented by a certain class
of connected DAGs labelled by a set of basic operations over stack trees
describing of the relative application positions of the basic
operations appearing on it. Applying a DAG to a stack tree t intuitively
amounts to paste its input vertices to some leaves of t and to simplify
the obtained structure, applying the basic operations labelling the edges of the
DAG to the leaves they are appended to, until either a new stack tree is
obtained or the process fails, in which case the application of the DAG to t
at the chosen position is deemed impossible. This model is a common extension
to those of higher-order stack operations presented by Carayol and of ground tree
transducers presented by Dauchet and Tison.

As further results we can define a notion of recognisable sets of
operations through a generalisation. The proof that the graphs generated
by a ground stack tree rewriting system have a decidable first order theory
with reachability is inspired by the technique of finite set interpretations
presented by Colcombet and Loding.

Abstract: Separation logic is used as an assertion language for
Hoare-style proof systems about programs with pointers,
and there is an ongoing quest for understanding its
complexity and expressive power. There are also
a lot of activities to develop verification methods
with decision procedures for fragments of practical use.

Actually, there exist many variants for separation
logic that can be viewed as fragments of second-order logic,
as well as variants of modal or temporal logics in which
models can be updated dynamically.

In this talk, after introducing first principles on separation logic,
issues related to decidability, computational complexity and expressive
power are discussed. We provide several tight relationships with
second-order logics, interval temporal logics or data logics, depending
on the variants of the logic and on the syntactic resources available.

Initial architects envinsioned RDF as a knowledge representation language, freeing users from syntactic limitations and revolutionizing the way information was exchanged.
While inference and description logic are applied to RDF, the foundation of simple assertions composed of global, unambiguous identifiers, has many more mondane and practical applications.
Distributed contributions to large (web-scale) data graphs demands adaptation of tree and stream-based validation techniques to operate over a graph.
Shape Expressions performs an ordered traversal of RDF graphs to
1 validate of structural constraints.
2 perform generative semantic actions.