Pentagon problem

Importance: High ✭✭✭
Author(s): Nesetril, Jaroslav
Subject: Graph Theory
» Coloring
» » Homomorphisms
Keywords: cubic
Recomm. for undergrads: no
Posted by: Robert Samal
on: March 24th, 2007
Question   Let $ G $ be a 3-regular graph that contains no cycle of length shorter than $ g $. Is it true that for large enough~$ g $ there is a homomorphism $ G \to C_5 $?

This question was asked by Nesetril at numerous problem sessions (and also appears as [N]). By Brook's theorem any triangle-free cubic graph is 3-colorable. Does a stronger assumption on girth of the graph imply stronger coloring properties?

This problem is motivated by complexity considerations [GHN] and also by exploration of density of the homomorphism order: We write $ G \prec H $ if there is a homomorphism $ G \to H $ but there is no homomorphism $ H \to G $. It is known that whenever $ G \prec H $ holds and $ H $~is not bipartite, there is a graph~$ K $ satisfying $ G \prec K \prec H $. A negative solution to the Pentagon problem would have the following density consequence: for each cubic graph~$ H $ for which~$ C_5 \prec H $ holds, there exists a cubic graph~$ K $ satisfying $ C_5 \prec K \prec H $ (see [N]).

If we replaced $ C_5 $ in the statement of the problem by a longer odd cycle, we would get a stronger statement. It is known that no such strenghthening is true. This was proved by Kostochka, Nesetril, and Smolikova [KNS] for $ C_{11} $ (hence for all $ C_l $ with $ l \ge 11 $), by Wanless and Wormald [WW] for $ C_9 $, and recently by Hatami [H] for $ C_7 $. Each of these results uses probabilistic arguments (random regular graphs), no constructive proof is known.

Haggkvist and Hell [HH] proved that for every integer~$ g $ there is a graph~$ U_g $ with odd girth at least~$ g $ (that is, $ U_g $ does not contain odd cycle of length less than~$ g $) such that every cubic graph of odd girth at least~$ g $ maps homomorphically to~$ U_g $. Here, the graph~$ U_g $ may have large degrees. This leads to a weaker version of the Pentagon problem:

Question   Is it true that for every $ k $ there exists a cubic graph $ H_k $ of girth~$ k $ and an integer~$ g $ such that every cubic graph of girth at least~$ g $ maps homomorphically to~$ H_k $?

A particular question in this direction: does a high-girth cubic graph map to the Petersen graph?

As an approach to this, we mention a result of DeVos and Samal [DS]: a cubic graph of girth at least~$ 17 $ admits a homomorphism to the Clebsch graph. In context of the Pentagon problem, the following reformulation is particularly appealing: If $ G $~is a cubic graph of girth at least~$ 17 $, then there is a cut-continuous mapping from~$ G $ to~$ C_5 $; that is, there is a mapping $ f: E(G) \to E(C_5) $ such that for any cut $ X \subseteq E(C_5) $ the preimage $ f^{-1}(X) $ is a cut. (Here by cut we mean the edge-set of a spanning bipartite subgraph. A more thorough exposition of cut-continuous mappings can be found in~[DNR].)


[DNR] Matt DeVos, Jaroslav Nesetril, and Andre Raspaud, On edge-maps whose inverse preverses flows and tensions, Graph Theory in Paris: Proceedings of a Conference in Memory of Claude Berge, Birkhäuser 2006.

[DS] Matt DeVos and Robert Samal, High girth cubic graphs map to the Clebsch graph

[GHN] Anna Galluccio, Pavol Hell, and Jaroslav Nesetril, The complexity of $ H $-colouring of bounded degree graphs, Discrete Math. 222 (2000), no.~1-3, 101--109, MathSciNet

[HH] Roland Haggkvist and Pavol Hell, Universality of $ A $-mote graphs, European J. Combin. 14 (1993), no.~1, 23--27.

[H] Hamed Hatami, Random cubic graphs are not homomorphic to the cycle of size~7, J. Combin. Theory Ser. B 93 (2005), no.~2, 319--325, MathSciNet

[KNS] Alexandr~V. Kostochka, Jaroslav Nesetril, and Petra Smolikova, Colorings and homomorphisms of degenerate and bounded degree graphs, Discrete Math. 233 (2001), no.~1-3, 257--276, Fifth Czech-Slovak International Symposium on Combinatorics, Graph Theory, Algorithms and Applications, (Prague, 1998), MathSciNet

*[N] Jaroslav Nesetril, Aspects of structural combinatorics (graph homomorphisms and their use), Taiwanese J. Math. 3 (1999), no.~4, 381--423, MathSciNet

[WW] I.M. Wanless and N.C. Wormald, Regular graphs with no homomorphisms onto cycles, J. Combin. Theory Ser. B 82 (2001), no.~1, 155--160, MathSciNet

* indicates original appearance(s) of problem.