On the number of cycles in a graph with restricted cycle lengths
  Let $L$ be a set of positive integers. We call a (directed) graph $G$ an $L$\emph{-cycle graph} if all cycle lengths in $G$ belong to $L$. Let $c(L,n)$ be the maximum number of cycles possible in an $n$-vertex $L$-cycle graph (we use $\vec{c}(L,n)$ for the number of cycles in directed graphs). In the undirected case we show that for any fixed set $L$, we have $c(L,n)=\Theta_L(n^{\lfloor k/\ell \rfloor})$ where $k$ is the largest element of $L$ and $2\ell$ is the smallest even element of $L$ (if $L$ contains only odd elements, then $c(L,n)=\Theta_L(n)$ holds.) We also give a characterization of $L$-cycle graphs when $L$ is a single element.   In the directed case we prove that for any fixed set $L$ we have $\vec{c}(L,n)=(1+o(1))(\frac{n-1}{k-1})^{k-1}$, where $k$ is the largest element of $L$. We determine the exact value of $\vec{c}(\{k\},n)$ for every $k$ and characterize all graphs attaining this maximum. 
D\'aniel Gerbner, Bal\'azs Keszegh, Cory Palmer, Bal\'azs Patk\'os
math.CO