VC density of set systems defnable in tree-like graphs

Abstract: We study set systems definable in graphs using variants of logic with different expressive power. Our focus is on the notion of Vapnik-Chervonenkis density: the smallest possible degree of a polynomial bounding the cardinalities of restrictions of such set systems. On one hand, we prove that if $\varphi(\bar x,\bar y)$ is a fixed CMSO$_1$ formula and $\cal C$ is a class of graphs with uniformly bounded cliquewidth, then the set systems defined by $\varphi$ in graphs from $\cal C$ have VC density at most $|\bar y|$, which is the smallest bound that one could expect. We also show an analogous statement for the case when $\varphi(\bar x,\bar y)$ is a CMSO$_2$ formula and $\cal C$ is a class of graphs with uniformly bounded treewidth. We complement these results by showing that if $\cal C$ has unbounded cliquewidth (respectively, treewidth), then, under some mild technical assumptions on $\cal C$, the set systems definable by CMSO$_1$ (respectively, CMSO$_2$) formulas in graphs from $\cal C$ may have unbounded VC dimension, hence also unbounded VC density.