# Number of Cliques in Minor-Closed Classes

 Importance: Medium ✭✭
 Author(s): Wood, David R.
 Subject: Graph Theory
 Keywords: clique graph minor
 Recomm. for undergrads: no
 Posted by: David Wood on: October 12th, 2009

\begin{question} Is there a constant $c$ such that every $n$-vertex $K_t$-minor-free graph has at most $c^tn$ cliques? \end{question}

Here a \emph{clique} is a (not neccessarily maximal) set of pairwise adjacent vertices in a graph.

See [RW, NSTW] for early bounds on the number of cliques. Wood [W] proved that the number of cliques in an $n$-vertex $K_t$-minor-free graph is at most $c^{t\sqrt{\log t}}n\enspace.$ Fomin et al. [FOT] improved this bound to $c^{t\log\log t}n\enspace.$

These results are based on the fact that every $n$-vertex $K_t$-minor-free graph has at most $ct\sqrt{\log t}n$ edges. This bound is tight for certain random graphs. So it is reasonable to expect that random graphs might also provide good lower bounds on the number of cliques.

Update 2014: Choongbum Lee and Sang-il Oum [LO] recently answered this question in the affirmative, and even proved it for excluded subdivisions. In particular, they proved that every $n$-vertex graph with no $K_t$-subdivision has at most $2^{474t}n$ cliques and also at most $2^{14t+o(t)}n$ cliques.

The question now is to determine the minimum constant. Wood [W] proved a lower bound of $3^{2t/3-o(t)}n$ using an appropriate sized complete graph minus a perfect matching. The same graph gives a lower bound of $3^{s-o(s)}n$ on the number of cliques in a graph with no $K_s$ subdivision.

## Bibliography

[FOT] Fedor V. Fomin, Sang il Oum, and Dimitrios M. Thilikos. \arxiv[Rank-width and tree-width of $H$-minor-free graphs]{math.CO/0910.0079}, \emph{European J. Combin.} 31 (7), 1617–1628, 2010.

[NSTW] Serguei Norine, Paul Seymour, Robin Thomas, Paul Wollan. Proper minor-closed families are small. \emph{J. Combin. Theory Ser. B}, 96(5):754--757, 2006.

[RW] Bruce Reed and David R. Wood. Fast separation in a graph with an excluded minor. In 2005 European Conf. on Combinatorics, Graph Theory and Applications (EuroComb '05), vol. AE of \emph{Discrete Math. Theor. Comput. Sci. Proceedings}, pp. 45--50. 2005.

* [W] David R. Wood. \arxiv[On the maximum number of cliques in a graph]{math.CO/0602191}. \emph{Graphs Combin.}, 23(3):337--352, 2007.

[LO] Choongbum Lee and Sang-il Oum. \arxiv[Number of cliques in graphs with forbidden minor]{1407.7707}, 2014.

* indicates original appearance(s) of problem.