Number of points in N^n of norm <= 2.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =20a(5) =36a(6) =63a(7) =106a(8) =171a(9) =265a(10) =396a(11) =573a(12) =806a(13) =1106a(14) =1485a(15) =1956a(16) =2533a(17) =3231a(18) =4066a(19) =5055a(20) =6216a(21) =7568a(22) =9131a(23) =10926a(24) =12975a(25) =15301a(26) =17928a(27) =20881a(28) =24186a(29) =27870