a(n) = 3*4^n - 2*3^n.

Terms

a(0) =1a(1) =6a(2) =30a(3) =138a(4) =606a(5) =2586a(6) =10830a(7) =44778a(8) =183486a(9) =747066a(10) =3027630a(11) =12228618a(12) =49268766a(13) =198137946a(14) =795740430a(15) =3192527658a(16) =12798808446a(17) =51281327226a(18) =205383589230a(19) =822309197898