Counting even cycles and even paths with bounded circumference

Abstract

For an integer $L$, write $C_{\ge L}$ for the family of cycles of length at least $L$. For $L=2a$ let $H(n,L)=K_a+\overline K_{n-a}$, and for $L=2a+1$ let $H(n,L)$ be obtained from $K_a+\overline K_{n-a}$ by adding one edge inside the independent part. We prove sharp results for two even target graphs, namely even cycles $C_{2s}$ and even paths $P_{2r+1}$. For even cycles, with $s\ge3$ and $L\ge2s$, we have \[ \mathrm{ex}(n,C_{2s},C_{\ge L+1})=C_{2s}(H(n,L)) \] for all sufficiently large $n$. Together with the known $C_4$ case of Zhu, Győri, He, Lv, Salia and Xiao~[Bull. Lond. Math. Soc. 55 (2023)], this verifies the even-cycle case of their conjecture on $\mathrm{ex}(n,C_k,C_{\ge L+1})$. For even paths, with $r\ge2$ and $L\ge2r$, we have \[ \mathrm{ex}(n,P_{2r+1},C_{\ge L+1})=N(P_{2r+1},H(n,L)) \] for all sufficiently large $n$. We also derive the corresponding exact results when the forbidden graph is a path $P_{p+1}$, sharpening the relevant even-cycle and even-path asymptotic results of Győri, Salia, Tompkins and Zamora~[Discrete Math. Theor. Comput. Sci. 21 no. 1 (2019)].