This page gathers a certain number of (not very well known) problems I was interested in at some point. I would be curious to see proofs or counterexamples if you find some.

## Equitable partition of planar graphs into 3 induced forests (2013, 2015)

An equitable partition of a graph G is a partition of the vertex-set of G such that the sizes of any two parts differ by at most one.

Is it true that any planar graph has an equitable partition into 3 induced forests?

In this paper we proved that for any k>=4, any planar graph has an equitable partition into k induced forests (this proved a conjecture formulated by Wu, Zhang, and Li in 2013).

## Coloring triangle-free planar graphs with bounded monochromatic components (2014)

In this paper we proved that planar graphs with bounded maximum degree can be colored with 3 colors, in such way that every color class is the union of connected components of bounded size.
A natural question is the following:

Is there a function f: NN rarr NN such that the vertices of every triangle-free planar graph with maximum degree Delta can be 2-colored in such a way that each monochromatic component has at most f(Delta) vertices?

## Two firefighters in planar graphs (2013)

In a graph G, a fire starts at some vertex. At every time step, firefighters can protect up to k vertices, and then the fire spreads to all unprotected neighbours. The k-surviving rate rho_k(G) of G is the expectation of the proportion of vertices that can be saved from the fire, if the starting vertex of the fire is chosen uniformly at random.
We conjectured in this paper that there is a constant epsilon>0, such that for any planar graph G, rho_2(G)>epsilon (i.e. two firefighters can save (in average) a constant proportion of the vertices of any planar graph). We proved the conjecture for triangle-free planar graphs and subsequently it was proved for any planar graphs without cycles of length i (for any fixed 4 <=i<=7).
This paper shows the following result: for any planar graph G, if 3 firefighters are available at the first round and then only 2 at each subsequent round, then a fraction of 2//21 of the vertices of G can be saved in average.

## Polynomial chi-boundedness (2012)

We say that a class of graphs cc F is chi-bounded if there is a function f such that for any graph G in cc F, chi(G) <= f(omega(G)), where chi(G) and omega(G) stand for the chromatic number and the clique number of G, respectively.
Is it true that if cc F is hereditary (i.e. closed under taking induced subgraphs), and chi-bounded, then there is a real c (depending on cc F) such that for any graph G in cc F, chi(G) <= omega(G)^c?

## Locally identifying coloring of chordal graphs (2012)

Let N[u] denote the closed neighborhood of a vertex u (i.e. u together with its neighbors).
We conjectured in this paper that every chordal graph with maximum clique size k can be properly colored with 2k colors in such a way that for any two adjacent vertices u and v with N[u]!=N[v], the set of colors appearing in N[u] is distinct from the set of colors appearing in N[v].
We proved the conjecture for several subclasses of chordal graphs: interval graphs, split graphs, and k-trees. Note that there are perfect graphs with clique number 3, requiring an arbitrary number of colors to make sure that for any two adjacent vertices u and v with N[u]!=N[v], the set of colors appearing in N[u] is distinct from the set of colors appearing in N[v].

## Constructive lower bounds for ayclic and star coloring

A proper coloring of the vertices of a graph G is acyclic if every cycle of G contains at least 3 colors. It was proved by Alon, McDiarmid and Reed in 1991 that graphs with maximum degree Delta have an acyclic coloring with O(Delta^{4//3}). They also gave a simple probabilistic argument showing that there exist graphs with maximum degree Delta with no acyclic coloring with c Delta^{4//3}//(log Delta)^{1//3} colors, for some constant c.
Interestingly, as far as I know, there is no deterministic construction of an infinite family of graphs with maximum degree Delta and with no acyclic coloring with c Delta colors, for some constant c>1 (let alone with Delta^{1+epsilon} colors, for some constant epsilon>0). This is hard to believe.

## Girth and density in hereditary classes (2009)

The girth of a graph G is the size of a smallest cycle of G, and the average degree of G is "ad"(G)= 2|E(G)|//|V(G)|.
Let cc F be a hereditary class of graphs (a class closed under taking induced subgraphs), and let cc "F"_g be the graphs of cc F of girth ast least g. I am interested in the following parameter: mu_{g}(cc F)="sup"_{G in cc "F"_g} "ad"(G). It is well known that for the family cc P of planar graphs, mu_{g}(cc P)= {2g}/{g-2} (this mainly follows from Euler's Formula). More generally, a classical result of Gallucio, Goddyn and Hell implies that in any proper minor-closed family cc F, mu_{g}(cc F) exists and tends to 2 as g->oo (see also this recent paper for a generalisation).
Interestingly, mu_{g}(cc F) is a rational number whenever cc F is the class of planar graphs, outerplanar graphs, partial 2-trees, but also the class SEG of intersection graphs of segments in the plane (when g>=5): it is not difficult to show that mu_{g}("SEG")= {2g-4}/{g-4} for any g>=5. But for the class CIRCLE of intersection graphs of the chords of a circle, things are quite different. We proved in this paper that mu_{g}("CIRCLE")= 2sqrt{{g-2}/{g-4}} for any g>=5.
This is fairly surprising and a bit mysterious. What other kind of function of g can you obtain? For instance, can you find a hereditary class cc F such that mu_{g}(cc F)= (some rational function of g)^{1//3}? Is it true that mu_{g}(cc F) is a rational function of g for graphs embeddable on a fixed surface? for any minor-closed class?

## Induced universal graphs for graphs with maximum degree at most two (2008-2016)

Let F_n be the class of graphs on n vertices and maximum degree at most two. What is the smallest number of vertices in a graph that contains all graphs of F_n as induced subgraphs? In this paper, we proved that the answer lies somewhere between 11 |__ n/6 __| and  5 |__ n/2 __|+25 . In this paper, it was proved that the answer is at most 2n-1. In two particular cases (when all the components are large, or all the components are small) they proved that the answer is a constant away from 11 |__ n/6 __|.

# Solved problems

## Coloring graphs on surfaces such that every monochromatic component has bounded size (2014)

We proved in this paper that every graph of Euler genus g is 5-list-colorable in such a way that every monochromatic component has size O(g). We conjectured that 5 can be replaced by 4 (this is known to be true for planar graphs, see this paper). A similar conjecture was made independently by Chappell and Gimbel for the chromatic number.
We also proved that graphs of Euler genus g and girth at least 6 are 2-list-colorable in such a way that every monochromatic component has size O(g). We conjectured that it should also be true for graphs of girth 5. This is still open for planar graphs. A related problem was posed independently by Axenovich, Ueckerdt, and Weiner in this paper.

The two conjectures were solved by Zdeněk Dvořák and Sergey Norin in this paper.