a(n) = (5*3^n + (-3)^n)/6.

Terms

a(0) =1a(1) =2a(2) =9a(3) =18a(4) =81a(5) =162a(6) =729a(7) =1458a(8) =6561a(9) =13122a(10) =59049a(11) =118098a(12) =531441a(13) =1062882a(14) =4782969a(15) =9565938a(16) =43046721a(17) =86093442a(18) =387420489a(19) =774840978a(20) =3486784401a(21) =6973568802a(22) =31381059609a(23) =62762119218a(24) =282429536481a(25) =564859072962