a(n) = (2^n - 1)*(4^n - 1).

Terms

a(0) =0a(1) =3a(2) =45a(3) =441a(4) =3825a(5) =31713a(6) =257985a(7) =2080641a(8) =16711425a(9) =133955073a(10) =1072692225a(11) =8585738241a(12) =68702695425a(13) =549688696833