a(n) = (9^n-4^n)/5.

Terms

a(0) =0a(1) =1a(2) =13a(3) =133a(4) =1261a(5) =11605a(6) =105469a(7) =953317a(8) =8596237a(9) =77431669a(10) =697147165a(11) =6275373061a(12) =56482551853a(13) =508359743893