a(0) = a(1) = a(2) = 0; a(n) = n!/(n-2) for n > 2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =12a(5) =40a(6) =180a(7) =1008a(8) =6720a(9) =51840a(10) =453600a(11) =4435200a(12) =47900160a(13) =566092800a(14) =7264857600a(15) =100590336000