Hitting forbidden induced subgraphs on bounded treewidth graphs

For a fixed graph $H$, the $H$-IS-Deletion problem asks, given a graph $G$, for the minimum size of a set $S \subseteq V(G)$ such that $G\setminus S$ does not contain $H$ as an induced subgraph. Motivated by previous work about hitting (topological) minors and subgraphs on bounded treewidth graphs, we are interested in determining, for a fixed graph $H$, the smallest function $fH(t)$ such that $H$-IS-Deletion can be solved in time $fH(t) \cdot n^{O(1)}$ assuming the Exponential Time Hypothesis (ETH), where $t$ and $n$ denote the treewidth and the number of vertices of the input graph, respectively. We show that $fH(t) = 2^{O(t{h-2})}$ for every graph $H$ on $h \geq 3$ vertices, and that $fH(t) = 2^{O(t)}$ if $H$ is a clique or an independent set. We present a number of lower bounds by generalizing a reduction of Cygan et al. [MFCS 2014] for the subgraph version. In particular, we show that when $H$ deviates slightly from a clique, the function $fH(t)$ suffers a sharp jump: if $H$ is obtained from a clique of size $h$ by removing one edge, then $fH(t) = 2^{{\Theta(t{h-2})}$.} We also show that $fH(t) = 2^{{\Omega(t{h})}$} when $H=K{h,h}$, and this reduction answers an open question of Mi. Pilipczuk [MFCS 2011] about the function $f{C4}(t)$ for the subgraph version. Motivated by Cygan et al. [MFCS 2014], we also consider the colorful variant of the problem, where each vertex of $G$ is colored with some color from $V(H)$ and we require to hit only induced copies of $H$ with matching colors. In this case, we determine, under the ETH, the function $fH(t)$ for every connected graph $H$ on $h$ vertices: if $h\leq 2$ the problem can be solved in polynomial time; if $h\geq 3$, $fH(t) = 2^{\Theta(t)}$ if $H$ is a clique, and $f_H(t) = 2^{{\Theta(t{h-2})}$} otherwise.