a(n) = (15^n - (-3)^n)/18.

Terms

a(0) =0a(1) =1a(2) =12a(3) =189a(4) =2808a(5) =42201a(6) =632772a(7) =9492309a(8) =142382448a(9) =2135743281a(10) =32036129532a(11) =480542002029