a(n) = 4^n + 5^n + 6^n.

Terms

a(0) =3a(1) =15a(2) =77a(3) =405a(4) =2177a(5) =11925a(6) =66377a(7) =374445a(8) =2135777a(9) =12292965a(10) =71280377a(11) =415819485a(12) =2437700177a(13) =14348506005a(14) =84736115177a(15) =501776304525