8n+3^n+5^n.

Terms

a(0) =2a(1) =16a(2) =50a(3) =176a(4) =738a(5) =3408a(6) =16402a(7) =80368a(8) =397250a(9) =1972880a(10) =9824754a(11) =49005360a(12) =244672162a(13) =1222297552a(14) =6108298706a(15) =30531927152a(16) =152630937474a(17) =763068593424