a(n) = (n + 1)^(n - 2)*(3*n + 1).

Terms

a(0) =1a(1) =2a(2) =7a(3) =40a(4) =325a(5) =3456a(6) =45619a(7) =720896a(8) =13286025a(9) =280000000a(10) =6645125311a(11) =175432531968