a(n) = 6*a(n-1) - 3*a(n-2), a(0)=1, a(1)=3.

Terms

a(0) =1a(1) =3a(2) =15a(3) =81a(4) =441a(5) =2403a(6) =13095a(7) =71361a(8) =388881a(9) =2119203a(10) =11548575a(11) =62933841a(12) =342957321a(13) =1868942403a(14) =10184782455a(15) =55501867521a(16) =302456857761