Generalized Turán problems for shorter even cycles

Abstract

For graphs $H$ and $F$, let $\text{ex}(n,H,F)$ denote the maximum number of copies of $H$ in an $n$-vertex $F$-free graph. Gerbner, Győri, Methuku, and Vizer proved that $\text{ex}(n,C_{2\ell},C_{2k})=Θ(n^\ell)$ for $k>\ell\ge2$. They determined the leading term for $\ell=2$, but for $k>\ell\ge3$ their general lower and upper bounds had different leading constants, leaving open the problem of closing this gap. We solve this problem by showing that, for every $k>\ell\ge2$, \[\text{ex}(n,C_{2\ell},C_{2k})=\left(\frac{(k-1)_\ell}{2\ell}+o(1)\right)n^\ell, \] where $(k-1)_\ell=(k-1)(k-2)\cdots(k-\ell)$.