a(n) = (3*5^n - 2*4^n + 3^n)/2.

Terms

a(0) =1a(1) =5a(2) =26a(3) =137a(4) =722a(5) =3785a(6) =19706a(7) =101897a(8) =523682a(9) =2677385a(10) =13629386a(11) =69136457a(12) =349699442a(13) =1764742985a(14) =8889229466a(15) =44709799817a(16) =224608392002