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

Terms

a(0) =0a(1) =3a(2) =6a(3) =36a(4) =120a(5) =528a(6) =2016a(7) =8256a(8) =32640a(9) =131328a(10) =523776a(11) =2098176a(12) =8386560a(13) =33558528a(14) =134209536a(15) =536887296a(16) =2147450880a(17) =8590000128a(18) =34359607296a(19) =137439215616a(20) =549755289600