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Samy Abbes

Publications and source records attributed to Samy Abbes.

At least 19 recordsLinked to original sources
arXiv2025

Configured spaces and their Möbius polynomials

An arbitrary dependence structure between a finite family of events of a probability space defines a hypergraph structure. We study the converse operation, starting from a hypergraph structure, to determine a canonical probability space with events indexed by the vertices and that satisfy the prescribed dependence structure; we also require that events that are not mutually independent must be mutually exclusive. Such a hypergraph structure, we call a configuration. Requiring furthermore that all events are given the same positive probability $t$, converts this problem to the study of a family of Möbius polynomials, which we call the relative Möbius polynomials (or independence polynomial) of the configuration. This study was very much already complete in the case where the configuration actually derives from a graph structure, but remained open in the general case. We show the existence and uniqueness of an optimal real $t$, optimal in the sense that the events cover a subset of maximal probability. We base our study on a new formula for the derivative of the Möbius polynomial.

math.CO↗
arXiv2025

Uniform generation of large traces

We introduce an algorithm for the uniform generation of infinite traces, i.e., infinite words up to commutation of some letters. The algorithm outputs on-the-fly approximations of a theoretical infinite trace, the latter being distributed according to the exact uniform probability measure. The average size of the approximation grows linearly with the time of execution of the algorithm, hence its output can be effectively used while running. Two versions of the algorithm are given. A version without rejection has a good production speed, provided that some precomputations have been done, but these may be costly. A version with rejection requires much fewer computations, at the expense of a production speed that can be small. We also show that, for some particular trace monoids, one or the other version of the algorithm can actually be very good: few computations for a good production speed.

math.CO↗
arXiv2025

Ergodic properties of concurrent systems

A concurrent system is defined as a monoid action of a trace monoid on a finite set of states. Concurrent systems represent state models where the state is distributed and where state changes are local. Starting from a spectral property on the combinatorics of concurrent systems, we prove the existence and uniqueness of a Markov measure on the space of infinite trajectories relatively to any weight distributions. In turn, we obtain a combinatorial result by proving that the kernel of the associated Möbius matrix has dimension 1; the Möbius matrix extends in this context the Möbius polynomial of a trace monoid. We study ergodic properties of irreducible concurrent systems and we prove a Strong law of large numbers. It allows us to introduce the speedup as a measurement of the average amount of concurrency within infinite trajectories. Examples are studied.

math.PR↗
arXiv2024

Convergence of distributions on paths

We study the convergence of distributions on finite paths of weighted digraphs, namely the family of Boltzmann distributions and the sequence of uniform distributions. Targeting applications to the convergence of distributions on paths, we revisit some known results from reducible nonnegative matrix theory and obtain new ones, with a systematic use of tools from analytic combinatorics. In several fields of mathematics, computer science and system theory, including concurreny theory, one frequently faces non strongly connected weighted digraphs encoding the elements of combinatorial structures of interest; this motivates our study.

math.PR↗
arXiv2022

Introduction to probabilistic concurrent systems

The first part of the paper is an introduction to the theory of probabilistic concurrent systems under a partial order semantics. Key definitions and results are given and illustrated on examples. The second part includes contributions. We introduce deterministic concurrent systems as a subclass of concurrent systems. Deterministic concurrent system are "locally commutative'" concurrent systems. We prove that irreducible and deterministic concurrent systems have a unique probabilistic dynamics, and we characterize these systems by means of their combinatorial properties.

cs.FL↗
arXiv2021

A spectral property for concurrent systems and some probabilistic applications

We study trace theoretic concurrent systems. This setting encompasses safe (1-bounded) Petri nets. We introduce a notion of irreducible concurrent system and we prove the equivalence between irreducibility and a "spectral property". The spectral property states a strict inequality between radii of convergence of certain growth series associated with the system. The proof that we present relies on analytic combinatorics techniques. The spectral property is the cornerstone of our theory, in a framework where the Perron-Frobenius theory does not apply directly. This restriction is an inherent difficulty in the study of concurrent systems. We apply the spectral property to the probabilistic theory of concurrent systems. We prove on the one hand the uniqueness of the uniform measure, a question left open in a previous paper. On the other hand, we prove that this uniform measure can be realized as a Markov chain of states-and-cliques on a state space that can be precisely characterized.

math.CO↗
arXiv2021

Deterministic concurrent systems

We introduce deterministic concurrent systems as a subclass of concurrent systems. Deterministic concurrent system are "locally commutative" concurrent systems. We prove that irreducible and deterministic concurrent systems have a unique probabilistic dynamics, and we characterise these systems by means of their analytic combinatorics properties.

math.CO↗
arXiv2019

Markovian dynamics of concurrent systems

Monoid actions of trace monoids over finite sets are powerful models of concurrent systems---for instance they encompass the class of 1-safe Petri nets. We characterise Markov measures attached to concurrent systems by finitely many parameters with suitable normalisation conditions. These conditions involve polynomials related to the combinatorics of the monoid and of the monoid action. These parameters generalise to concurrent systems the coefficients of the transition matrix of a Markov chain. A natural problem is the existence of the uniform measure for every concurrent system. We prove this existence under an irreducibility condition. The uniform measure of a concurrent system is characterised by a real number, the characteristic root of the action, and a function of pairs of states, the Parry cocyle. A new combinatorial inversion formula allows to identify a polynomial of which the characteristic root is the smallest positive root. Examples based on simple combinatorial tilings are studied.

math.CO↗
arXiv2017

Uniform generation of infinite concurrent runs: the case of trace monoids

We introduce an algorithm for the uniform generation of infinite runs in concurrent systems under a partial order probabilistic semantics. We work with trace monoids as concurrency models. The algorithm outputs on-the-fly approximations of a theoretical infinite run, the latter being distributed according to the exact uniform probability measure. The average size of the approximation grows linearly with the time of execution of the algorithm. The execution of the algorithm only involves distributed computations, provided that some - costly - precomputations have been done.

math.CO↗
arXiv2017

Toward uniform random generation in 1-safe Petri nets

We study the notion of uniform measure on the space of infinite executions of a 1-safe Petri net. Here, executions of 1-safe Petri nets are understood up to commutation of concurrent transitions, which introduces a challenge compared to usual transition systems. We obtain that the random generation of infinite executions reduces to the simulation of a finite state Markov chain. Algorithmic issues are discussed.

cs.FL↗
arXiv2016

Synchronization of Bernoulli sequences on shared letters

The topic of this paper is the distributed and incremental generation of long executions of concurrent systems, uniformly or more generally with weights associated to elementary actions. Synchronizing sequences of letters on alphabets sharing letters are known to produce a trace in the concurrency theoretic sense, i.e., a labeled partially ordered set. We study the probabilistic aspects by considering the synchronization of Bernoulli sequences of letters, under the light of Bernoulli and uniform measures recently introduced for trace monoids. We introduce two algorithms that produce random traces, using only local random primitives. We thoroughly study some specific examples, the path model and the ring model, both of arbitrary size. For these models, we show how to generate any Bernoulli distributed random traces, which includes the case of uniform generation.

cs.FL↗
arXiv2016

Uniform measures on braid monoids and dual braid monoids

We aim at studying the asymptotic properties of typical positive braids, respectively positive dual braids. Denoting by $μ_k$ the uniform distribution on positive (dual) braids of length $k$, we prove that the sequence $(μ_k)_k$ converges to a unique probability measure $μ_{\infty}$ on infinite positive (dual) braids. The key point is that the limiting measure $μ_{\infty}$ has a Markovian structure which can be described explicitly using the combinatorial properties of braids encapsulated in the Möbius polynomial. As a by-product, we settle a conjecture by Gebhardt and Tawn (J. Algebra, 2014) on the shape of the Garside normal form of large uniform braids.

math.GR↗
arXiv2015

Uniform generation in trace monoids

We consider the problem of random uniform generation of traces (the elements of a free partially commutative monoid) in light of the uniform measure on the boundary at infinity of the associated monoid. We obtain a product decomposition of the uniform measure at infinity if the trace monoid has several irreducible components-a case where other notions such as Parry measures, are not defined. Random generation algorithms are then examined.

cs.FL↗
arXiv2015

Uniform and Bernoulli measures on the boundary of trace monoids

Trace monoids and heaps of pieces appear in various contexts in combinatorics. They also constitute a model used in computer science to describe the executions of asynchronous systems. The design of a natural probabilistic layer on top of the model has been a long standing challenge. The difficulty comes from the presence of commuting pieces and from the absence of a global clock. In this paper, we introduce and study the class of Bernoulli probability measures that we claim to be the simplest adequate probability measures on infinite traces. For this, we strongly rely on the theory of trace combinatorics with the Möbius polynomial in the key role. These new measures provide a theoretical foundation for the probabilistic study of concurrent systems.

math.CO↗
arXiv2015

A cut-invariant law of large numbers for random heaps

Heap monoids equipped with Bernoulli measures are a model of probabilistic asynchronous systems. We introduce in this framework the notion of asynchronous stopping time, which is analogous to the notion of stopping time for classical probabilistic processes. A Strong Bernoulli property is proved. A notion of cut-invariance is formulated for convergent ergodic means. Then a version of the Strong law of large numbers is proved for heap monoids with Bernoulli measures. Finally, we study a sub-additive version of the Law of large numbers in this framework based on Kingman sub-additive Ergodic Theorem.

math.CO↗
arXiv2015

Representation of traces by vectors of words

We show that every trace monoid is isomorphic to a sub-monoid of a monoid of word vectors. It provides a concrete representation of the elements of a trace monoid as processes associated with a resource sharing mechanism. We illustrate this representation by obtaining some results on the ordering structure of the left divisibility relation on trace monoids.

cs.DM↗
arXiv2015

A Graded Möbius transform and its harmonic interpretation

We give a graded version of the Möbius inversion formula in the framework of trace monoids. The formula is based on a graded version of the Möbius transform, related to the notion of height deriving from the Cartier-Foata normal form of the elements of a trace monoid. Using the notion of Bernoulli measures on the boundary of a trace monoid developped recently, we study a probabilistic interpretation of the graded inversion formula. We introduce Möbius harmonic functions for trace monoids and obtain an integral representation formula for them, analogous to the Poisson formula for harmonic functions associated to random walks on trees.

math.CO↗