a(n) = 6^n + 7^n + 8^n.

Terms

a(0) =3a(1) =21a(2) =149a(3) =1071a(4) =7793a(5) =57351a(6) =426449a(7) =3200631a(8) =24221633a(9) =184649031a(10) =1416683249a(11) =10930058391a(12) =84737546273a(13) =659705518311