a(n) = 1^n + 2^n + 4^n.

Terms

a(0) =3a(1) =7a(2) =21a(3) =73a(4) =273a(5) =1057a(6) =4161a(7) =16513a(8) =65793a(9) =262657a(10) =1049601a(11) =4196353a(12) =16781313a(13) =67117057a(14) =268451841a(15) =1073774593a(16) =4295032833a(17) =17180000257a(18) =68719738881a(19) =274878431233