a(n) = 3^n + 5^n.

Terms

a(0) =2a(1) =8a(2) =34a(3) =152a(4) =706a(5) =3368a(6) =16354a(7) =80312a(8) =397186a(9) =1972808a(10) =9824674a(11) =49005272a(12) =244672066a(13) =1222297448a(14) =6108298594a(15) =30531927032a(16) =152630937346a(17) =763068593288