a(n) = 7^n + 5^n + 3^n + 2^n.

Terms

a(0) =4a(1) =17a(2) =87a(3) =503a(4) =3123a(5) =20207a(6) =134067a(7) =903983a(8) =6162243a(9) =42326927a(10) =292300947a(11) =2026334063a(12) =14085963363a(13) =98111316047a(14) =684331387827