a(n) = (3^n + 2*3^(n/2)*cos(n*Pi/6))/3.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =24a(5) =72a(6) =225a(7) =702a(8) =2160a(9) =6561a(10) =19764a(11) =59292a(12) =177633a(13) =532170a(14) =1595052a(15) =4782969a(16) =14346720a(17) =43040160a(18) =129127041a(19) =387400806a(20) =1162241784a(21) =3486784401a(22) =10460412252a(23) =31381236756a(24) =94143533121a(25) =282430067922a(26) =847289140884