a(n) = (7*3^n - (-3)^n)/6.

Terms

a(0) =1a(1) =4a(2) =9a(3) =36a(4) =81a(5) =324a(6) =729a(7) =2916a(8) =6561a(9) =26244a(10) =59049a(11) =236196a(12) =531441a(13) =2125764a(14) =4782969a(15) =19131876a(16) =43046721a(17) =172186884a(18) =387420489a(19) =1549681956a(20) =3486784401a(21) =13947137604a(22) =31381059609a(23) =125524238436a(24) =282429536481