a(n) = 1^n + 3^n + 7^n.

Terms

a(0) =3a(1) =11a(2) =59a(3) =371a(4) =2483a(5) =17051a(6) =118379a(7) =825731a(8) =5771363a(9) =40373291a(10) =282534299a(11) =1977503891a(12) =13841818643a(13) =96890604731a(14) =678227855819