a(n) = n!*(3*n-1)/2.

Terms

a(0) =1a(1) =5a(2) =24a(3) =132a(4) =840a(5) =6120a(6) =50400a(7) =463680a(8) =4717440a(9) =52617600a(10) =638668800a(11) =8382528000a(12) =118313395200