a(1) = 0, a(2) = 4, a(3) = 12; for n > 3, a(n) = 3*a(n-1) - 3*a(n-2) + 9*a(n-3).

Terms

a(0) =0a(1) =4a(2) =12a(3) =24a(4) =72a(5) =252a(6) =756a(7) =2160a(8) =6480a(9) =19764a(10) =59292a(11) =176904a(12) =530712a(13) =1595052a(14) =4785156a(15) =14346720a(16) =43040160a(17) =129146724a(18) =387440172a(19) =1162241784a(20) =3486725352a(21) =10460412252a(22) =31381236756a(23) =94143001680a(24) =282429005040a(25) =847289140884