a(n) = 2^n + 4^n + 7^n.

Terms

a(0) =3a(1) =13a(2) =69a(3) =415a(4) =2673a(5) =17863a(6) =121809a(7) =840055a(8) =5830593a(9) =40616263a(10) =283524849a(11) =1981523095a(12) =13858068513a(13) =96956127463a(14) =678491524689