\(C_n = \binom{2n}{n} - \binom{2n}{n+1}\), which stays in exact integers and never divides. Counts balanced bracket sequences, triangulations of a convex \((n+2)\)-gon and binary trees on \(n\) nodes, among many others.
\(C_n = \binom{2n}{n} - \binom{2n}{n+1}\), which stays in exact integers and never divides. Counts balanced bracket sequences, triangulations of a convex \((n+2)\)-gon and binary trees on \(n\) nodes, among many others.