a(0)=1, a(1)=3, a(n) = 4*a(n-1) + 3*a(n-2) for n > 1.

Terms

a(0) =1a(1) =3a(2) =15a(3) =69a(4) =321a(5) =1491a(6) =6927a(7) =32181a(8) =149505a(9) =694563a(10) =3226767a(11) =14990757a(12) =69643329a(13) =323545587a(14) =1503112335a(15) =6983086101a(16) =32441681409a(17) =150715983939a(18) =700188979983