Retour à la page principale.

Orateurs: O. Angel (Toronto), N. Broutin (Montréal), J. Bouttier (Saclay), B. Eynard (Saclay), I. Gessel (Brandeis), T. Guttmann (Melbourne), M. Krikun (Nancy), J.F. Le Gall (Paris), S. Vidal (Lille), N. Wormald (Waterloo)

Soit 10 exposés de 50 minutes + questions.

Programme

- 09h55-10h00 : accueil
- 10h00-11h00 : J. Bouttier, Counting maps with trees, for fun and profit.
- 11h15-12h15 : O. Angel, Recurrence of weak graph limits.

- 14h00-15h00 : N. Wormald, Enumeration of unrooted planar maps.
- 15h00-16h00 : N. Broutin, Heights of random trees: towards unification.
- 16h30-17h30 : B. Eynard, Number of maps of any genus, algebraic geometry and integrable hierarchies.

- 10h00-11h00 : J.F. Le Gall, La limite continue des grandes cartes planaires aléatoires.
- 11h15-12h15 : M. Krikun, Triangulations with multiple boundaries.

- 14h00-15h00 : I. Gessel, Is Analysis Necessary?
- 15h00-16h00 : S. Vidal, Combinatorial Maps, Modular Group and the Airy Function.
- 16h30-17h30 : T. Guttmann, Area Distribution and Scaling Functions for Polygons.

**J. Bouttier**, Counting maps with trees,
for fun and profit.

I will review a number of bijections relating planar maps to trees, that
were first devised as elementary combinatorial proofs for many map
enumeration results alternatively found by more involved techniques
(typically requiring to "solve" Tutte equations or matrix models), but
then turned out to have interesting applications in the study of random
maps.

**O. Angel**, Recurrence of weak graph limits.

A result of Benjamini and Schramm states that the weak limit of a
sequence of planar graphs with uniformly bounded degrees is a.s.
recurrent. We extend this results to limits of graphs excluding (any)
fixed minor. Joint with Balazs Szegedy.

**N. Wormald**, Enumeration of unrooted planar maps.

I will discuss various aspects of the enumeration of unrooted
planar maps. This will include both exact and asymptotic results, and in
particular some description of how to efficiently enumerate unrooted
3-connected planar maps (i.e. unlabelled 3-connected planar graphs).

**N. Broutin**,
Heights of random trees: towards unification.

(Joint work with L. Devroye, E. McLeish, and M. De la Salle)

Problems related to lengths of paths in trees appear in applications
of Computer Science (analysis of algorithms), theoretical physics
(first passage percolation), and probability (branching random
walks). We aim at studying the random trees of logarithmic height in
a unified way. For this class of trees, laws of large numbers for
depths and heights in random trees can be obtained using an embedding
using branching random walks and large deviations. However, some
classes of trees, like most kinds of increasing trees, cannot be
constructed using the latter embedding. To cope with this issue, we
introduce a model based on a generalized branching random walk that
premits to capture more many new kinds of trees. We will review
classical examples using this approach, and will describe how the
framework can be used to obtain various new results explaining the
structure of random trees.

**M. Krikun**, Triangulations with multiple boundaries.

The combinatorial (i.e. not involving matrix integrals) methods
of map enumeration, when applied to maps on surfaces of higher
genera, lead naturally to the problem of enumeration for surfaces
with multiple holes.
I will remind the general method and related results, and then
concentrate on a simpler example -- certain class of triangulations,
where the exact formulas can be obtained in the planar case.
Finally, I will discuss possible applications of this result.

**J.F. Le Gall**,
La limite continue des grandes cartes planaires aléatoires.

L'ensemble des sommets d'une carte planaire, muni de la distance
de graphe, est un espace métrique fini. Si on choisit la carte planaire
au hasard parmi les cartes à n faces dans une classe
convenable, et si on renormalise la distance par le facteur
n^{-1/4}, on conjecture que l'espace métrique aléatoire obtenu
converge en loi vers un espace métrique compact aléatoire
parfois appelé la carte brownienne. Une premiére version
de cette convergence a été obtenue par Marckert et Mokkadem
dans un sens faible et pour le cas des quadrangulations. Nous discutons
des progrés récents concernant le cas de cartes biparties et traitant
la convergence au sens de la métrique de
Gromov-Hausdorff utilisée par les géométres.
En application, nous montrons que toute limite continue
des grandes cartes planaires aléatoires est de dimension de Hausdorff
4 et homéomorphe a la sphére de dimension deux.

**B. Eynard**,
Number of maps of any genus, algebraic geometry and integrable hierarchies.

Using formal matrix integrals, it is possible to write recursion
relations called loop equations (Tutte's equations for the simplest
cases) for counting various types of maps (arbitrary face degrees,
possibly bi-colored, with boundaries,...).
We present the solution (for any genus), in terms of algebraic geometry
invariants of a rational spectral curve, i.e. residues of rational
functions. Moreover, the algebraic invariants of that spectral curve
satisfy an integrable hierarchy (multicomponent KP).
The solution also allows to find the "continuum limits" of map numbers.

**I. Gessel** Is Analysis Necessary?

Abstract: Analytic techniques can be used to prove results, such as
combinatorial identities or theorems about formal power series, whose
statement involves no analysis. I will discuss the question of
whether analysis is really necessary to prove such results, with
examples involving integration, residues, P-recursive sequences, and
asymptotic series.

**S. Vidal**,
Combinatorial Maps, Modular Group and the Airy Function.

We shall present some recent results in exact enumeration of unrooted
maps. They use as a major ingredient, the theory of combinatorial
species due to Joyal and the combinatorics school of Montreal.
Incidentally, those enumeration results, revealed a exciting new
connection with the asymptotic of the Airy function. We shall give a
precise statement and a combinatorial proof of it. The method of proof
is combinatorial in that it use an interesting recursive decomposition
of pointed triangular maps. The immediate application of this
recursive decomposition are, a simple and optimal algorithm of
exhaustive generation and an unbiased random sampler. In the
introduction we shall insist on the group-theoretic aspects of
combinatorial maps which are the main feature of our approach.

**T. Guttmann**,
Area Distribution and Scaling Functions for Polygons.

In joint work with Christoph Richard and Iwan Jensen, we study the
area distribution and scaling function of self-avoiding polygons
"punctured" by holes which are also self-avoiding polygons. Similarly
for staircase polygons, punctured by staircase polygonal holes. We
prove some theorems, conjecture some exact results, provide extensive
numerical support for our conjectures, and discuss limit
distributions and scaling functions.