a(n) = 2^n + 6^n + 7^n.

Terms

a(0) =3a(1) =15a(2) =89a(3) =567a(4) =3713a(5) =24615a(6) =164369a(7) =1103607a(8) =7444673a(9) =50431815a(10) =342942449a(11) =2340125847a(12) =16018073633a(13) =109949712615a(14) =756587253329