a(n) = 5^n+3^n-1.

Terms

a(0) =1a(1) =7a(2) =33a(3) =151a(4) =705a(5) =3367a(6) =16353a(7) =80311a(8) =397185a(9) =1972807a(10) =9824673a(11) =49005271a(12) =244672065a(13) =1222297447a(14) =6108298593a(15) =30531927031a(16) =152630937345a(17) =763068593287