a(n) = Sum_{k=0..n-1} C(n,k+1)*C(n,k)*C(n-1,k).

Terms

a(0) =0a(1) =1a(2) =4a(3) =24a(4) =152a(5) =1010a(6) =6912a(7) =48328a(8) =343408a(9) =2471274a(10) =17966360a(11) =131717960a(12) =972488640a(13) =7223061040a(14) =53925450880a(15) =404400203280