Number of points in N^n of norm <= 4.

Terms

a(0) =1a(1) =5a(2) =17a(3) =54a(4) =165a(5) =482a(6) =1319a(7) =3390a(8) =8262a(9) =19250a(10) =43154a(11) =93601a(12) =197381a(13) =406200a(14) =817954a(15) =1614348a(16) =3126068a(17) =5943728a(18) =11103912a(19) =20396957a(20) =36869790a(21) =65637607a(22) =115178814a(23) =199377387a(24) =340708159a(25) =575150851a(26) =959691775a(27) =1583660542