3^n+5^n-2n.

Terms

a(0) =2a(1) =6a(2) =30a(3) =146a(4) =698a(5) =3358a(6) =16342a(7) =80298a(8) =397170a(9) =1972790a(10) =9824654a(11) =49005250a(12) =244672042a(13) =1222297422a(14) =6108298566a(15) =30531927002a(16) =152630937314a(17) =763068593254