a(n) = 5^n - 2^n.

Terms

a(0) =0a(1) =3a(2) =21a(3) =117a(4) =609a(5) =3093a(6) =15561a(7) =77997a(8) =390369a(9) =1952613a(10) =9764601a(11) =48826077a(12) =244136529a(13) =1220694933a(14) =6103499241a(15) =30517545357a(16) =152587825089a(17) =762939322053