28 research outputs found
Master index
Pla general, del mural cerĂ mic que decora una de les parets del vestĂbul de la Facultat de QuĂmica de la UB. El mural representa diversos sĂmbols relacionats amb la quĂmica
A counterexample to Thiagarajan's conjecture on regular event structures
We provide a counterexample to a conjecture by Thiagarajan (1996 and 2002)
that regular event structures correspond exactly to event structures obtained
as unfoldings of finite 1-safe Petri nets. The same counterexample is used to
disprove a closely related conjecture by Badouel, Darondeau, and Raoult (1999)
that domains of regular event structures with bounded -cliques are
recognizable by finite trace automata. Event structures, trace automata, and
Petri nets are fundamental models in concurrency theory. There exist nice
interpretations of these structures as combinatorial and geometric objects.
Namely, from a graph theoretical point of view, the domains of prime event
structures correspond exactly to median graphs; from a geometric point of view,
these domains are in bijection with CAT(0) cube complexes.
A necessary condition for both conjectures to be true is that domains of
regular event structures (with bounded -cliques) admit a regular nice
labeling. To disprove these conjectures, we describe a regular event domain
(with bounded -cliques) that does not admit a regular nice labeling.
Our counterexample is derived from an example by Wise (1996 and 2007) of a
nonpositively curved square complex whose universal cover is a CAT(0) square
complex containing a particular plane with an aperiodic tiling. We prove that
other counterexamples to Thiagarajan's conjecture arise from aperiodic 4-way
deterministic tile sets of Kari and Papasoglu (1999) and Lukkarila (2009).
On the positive side, using breakthrough results by Agol (2013) and Haglund
and Wise (2008, 2012) from geometric group theory, we prove that Thiagarajan's
conjecture is true for regular event structures whose domains occur as
principal filters of hyperbolic CAT(0) cube complexes which are universal
covers of finite nonpositively curved cube complexes
Enumerative Combinatorics
Enumerative Combinatorics focusses on the exact and asymptotic counting of combinatorial objects. It is strongly connected to the probabilistic analysis of large combinatorial structures and has fruitful connections to several disciplines, including statistical physics, algebraic combinatorics, graph theory and computer science. This workshop brought together experts from all these various fields, including also computer algebra, with the goal of promoting cooperation and interaction among researchers with largely varying backgrounds
Exact solution of the dimer model: Corner free energy, correlation functions and combinatorics
In this work, some classical results of the pfaffian theory of the dimer
model based on the work of Kasteleyn, Fisher and Temperley are introduced in a
fermionic framework. Then we shall detail the bosonic formulation of the model
{\it via} the so-called height mapping and the nature of boundary conditions is
unravelled. The complete and detailed fermionic solution of the dimer model on
the square lattice with an arbitrary number of monomers is presented, and
finite size effect analysis is performed to study surface and corner effects,
leading to the extrapolation of the central charge of the model. The solution
allows for exact calculations of monomer and dimer correlation functions in the
discrete level and the scaling behavior can be inferred in order to find the
set of scaling dimensions and compare to the bosonic theory which predict
particular features concerning corner behaviors. Finally, some combinatorial
and numerical properties of partition functions with boundary monomers are
discussed, proved and checked with enumeration algorithms.Comment: Final version to be published in Nuclear Physics B (53 pages and a
lot of figures
Proceedings of the 8th Cologne-Twente Workshop on Graphs and Combinatorial Optimization
International audienceThe Cologne-Twente Workshop (CTW) on Graphs and Combinatorial Optimization started off as a series of workshops organized bi-annually by either Köln University or Twente University. As its importance grew over time, it re-centered its geographical focus by including northern Italy (CTW04 in Menaggio, on the lake Como and CTW08 in Gargnano, on the Garda lake). This year, CTW (in its eighth edition) will be staged in France for the first time: more precisely in the heart of Paris, at the Conservatoire National d’Arts et Métiers (CNAM), between 2nd and 4th June 2009, by a mixed organizing committee with members from LIX, Ecole Polytechnique and CEDRIC, CNAM
Advanced Determinant Calculus: A Complement
This is a complement to my previous article "Advanced Determinant Calculus"
(S\'eminaire Lotharingien Combin. 42 (1999), Article B42q, 67 pp.). In the
present article, I share with the reader my experience of applying the methods
described in the previous article in order to solve a particular problem from
number theory (G. Almkvist, J. Petersson and the author, Experiment. Math. 12
(2003), 441-456). Moreover, I add a list of determinant evaluations which I
consider as interesting, which have been found since the appearance of the
previous article, or which I failed to mention there, including several
conjectures and open problems.Comment: AmS-LaTeX, 85 pages; Final, largely revised versio
Algebraic combinatorics of graph spectra, subspace arrangements and Tutte polynomials
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1996.Includes bibliographical references (p. 141-145).by Christos A. Athanasiadis.Ph.D