a(n) = (n!)^2 * Sum_{k=0..n} (2*k)! / (2^k * (k!)^3 * (n - k)!).

A331817

a(n) = (n!)^2 * Sum_{k=0..n} (2*k)! / (2^k * (k!)^3 * (n - k)!).

Terms

    a(0) =1a(1) =2a(2) =9a(3) =66a(4) =681a(5) =9090a(6) =148905a(7) =2889810a(8) =64805265a(9) =1648535490a(10) =46896669225

External references