Counting triangles in graphs without suspensions of paths

Abstract

For a graph $H$, the suspension $\widehat H$ of $H$ is a graph obtained from $H$ by joining a new vertex to every vertex of $H$. The generalized Tur\'an number $\textrm{ex}(n,K_3,\widehat H)$ is the maximum number of triangles in an $n$-vertex $\widehat H$-free graph. For fixed $k \ge 4$, Mubayi and Mukherjee conjectured that \[ \textrm{ex}(n,K_3,\widehat P_k) =\left\lfloor\frac{k-2}{2}\right\rfloor\frac{n^2}{8}+o(n^2), \] as $n \to \infty$, where $P_k$ is the path on $k$ vertices. They verified the conjecture for $k=4,5,6$. In this paper, we determine the asymptotic value of $\textrm{ex}(n,K_3,\widehat P_k)$ for every $k\ge 4$ via a unified approach and thus prove the conjecture of Mubayi and Mukherjee.