a(n) = 6^n - 3^n.

Terms

a(0) =0a(1) =3a(2) =27a(3) =189a(4) =1215a(5) =7533a(6) =45927a(7) =277749a(8) =1673055a(9) =10058013a(10) =60407127a(11) =362619909a(12) =2176250895a(13) =13059099693a(14) =78359381127a(15) =470170635669