Tur\'an numbers of cycles plus a general graph

Abstract

For a family of graphs $\cal F$, a graph $G$ is $\cal F$-free if it does not contain a member of $\cal F$ as a subgraph. The Tur\'an number $\textrm{ex}(n,{\cal F})$ is the maximum number of edges in an $n$-vertex graph which is $\cal F$-free. Let ${\cal C}_{\geq k}$ be the set of cycles with length at least $k$. In this paper, we investigate the Tur\'an number of $\{{\cal C}_{\geq k}, F\}$ for a general graph $F$. To be precise, we determine $\textrm{ex}(n, \{{\cal C}_{\geq k}, F\})$ apart from a constant additive term, where $F$ either is a 2-connected nonbipartite graph or is a 2-connected bipartite graph under some conditions. This is an extension of a previous result on the Tur\'an number of $\{{\cal C}_{\geq k}, K_r\}$ by the first author, Ning, and the third author.