a(n) = (3^n+1)*(3^(n+1)+1)/4.

Terms

a(0) =2a(1) =10a(2) =70a(3) =574a(4) =5002a(5) =44530a(6) =399310a(7) =3589414a(8) =32291602a(9) =290585050a(10) =2615147350a(11) =23535971854a(12) =211822683802