a(0) = 1; for n > 0, a(n) = (n!*(3*n+1))/2.

Terms

a(0) =1a(1) =2a(2) =7a(3) =30a(4) =156a(5) =960a(6) =6840a(7) =55440a(8) =504000a(9) =5080320a(10) =56246400a(11) =678585600a(12) =8861529600a(13) =124540416000