Few $T$ copies in $H$-saturated graphs

Abstract

A graph is $F$-saturated if it is $F$-free but the addition of any edge creates a copy of $F$. In this paper we study the quantity $\mathrm{sat}(n, H, F)$ which denotes the minimum number of copies of $H$ that an $F$-saturated graph on $n$ vertices may contain. This parameter is a natural saturation analogue of Alon and Shikhelman's generalized Tur\'an problem, and letting $H = K_2$ recovers the well-studied saturation function. We provide a first investigation into this general function focusing on the cases where the host graph is either $K_s$ or $C_k$-saturated. Some representative interesting behavior is: (a) For any natural number $m$, there are graphs $H$ and $F$ such that $\mathrm{sat}(n, H, F) = \Theta(n^m)$. (b) For many pairs $k$ and $l$, we show $\mathrm{sat}(n, C_l, C_k) = 0$. In particular, we prove that there exists a triangle-free $C_k$-saturated graph on $n$ vertices for any $k > 4$ and large enough $n$. (c) $\mathrm{sat}(n, K_3, K_4) = n-2$, $\mathrm{sat}(n, C_4, K_4) \sim \frac{n^2}{2}$, and $\mathrm{sat}(n, C_6, K_5) \sim n^3$. We discuss several intriguing problems which remain unsolved.