From Graph Properties to Graph Parameters: Tight Bounds for Counting on Small Subgraphs

A graph property is a function $Φ$ that maps every graph to {0, 1} and is invariant under isomorphism. In the $\#IndSub(Φ)$ problem, given a graph $G$ and an integer $k$, the task is to count the number of $k$-vertex induced subgraphs $G'$ with $Φ(G')=1$. $\#IndSub(Φ)$ can be naturally generalized to graph parameters, that is, to functions $Φ$ on graphs that do not necessarily map to {0, 1}: now the task is to compute the sum $\sum_{G'} Φ(G')$ taken over all $k$-vertex induced subgraphs $G'$. This problem setting can express a wider range of counting problems (for instance, counting $k$-cycles or $k$-matchings) and can model problems involving expected values (for instance, the expected number of components in a subgraph induced by $k$ random vertices). Our main results are lower bounds on $\#IndSub(Φ)$ in this setting, which simplify, generalize, and tighten the recent lower bounds of Döring, Marx, and Wellnitz [STOC'24] in various ways. (1) We show a lower bound for every nontrivial edge-monotone graph parameter $Φ$ with finite codomain (not only for parameters that take value in {0, 1}). (2) The lower bound is tight: we show that, assuming ETH, there is no $f(k)n^{o(k)}$ time algorithm. (3) The lower bound applies also to the modular counting versions of the problem. (4) The lower bound applies also to the multicolored version of the problem. We can extend the #W[1]-hardness result to the case when the codomain of $Φ$ is not finite, but has size at most $(1 - \varepsilon)\sqrt{k}$ on $k$-vertex graphs. However, if there is no bound on the size of the codomain, the situation changes significantly: for example, there is a nontrivial edge-monotone function $Φ$ where the size of the codomain is $k$ on $k$-vertex graphs and $\#IndSub(Φ)$ is FPT.