a(n) = 5^n + 6^n + 7^n.

Terms

a(0) =3a(1) =18a(2) =110a(3) =684a(4) =4322a(5) =27708a(6) =179930a(7) =1181604a(8) =7835042a(9) =52384428a(10) =352707050a(11) =2388951924a(12) =16262210162a(13) =111170407548a(14) =762690752570