The Wilf–Stanley exponential-growth conjecture for permutation classes

Let $X$ be a non-empty set of permutations, and let $s_n(X)=\left|\operatorname{Av}(X)\cap\mathcal{S}_n\right|$, where $\operatorname{Av}(X)$ is the class of permutations avoiding every element of $X$. Wilf–Stanley conjecture. There exists a real number $c_X$ such that

$$s_n(X)\leq c_X^n.$$

The conjecture asserts exponential boundedness of the enumeration sequence of every nonempty permutation class defined by avoidance. It was resolved by Marcus and Tardos, so the claim is solved.