a(n) = 1^n + 7^n + 8^n.

Terms

a(0) =3a(1) =16a(2) =114a(3) =856a(4) =6498a(5) =49576a(6) =379794a(7) =2920696a(8) =22542018a(9) =174571336a(10) =1356217074a(11) =10567261336a(12) =82560763938a(13) =646644824296