An approximation to population of x^2 + y^2 <= 2^n.

Terms

a(0) =1a(1) =3a(2) =4a(3) =5a(4) =9a(5) =15a(6) =27a(7) =50a(8) =92a(9) =171a(10) =322a(11) =610a(12) =1161a(13) =2220a(14) =4260a(15) =8201a(16) =15828a(17) =30622a(18) =59362a(19) =115287a(20) =224260a(21) =436871a(22) =852161a(23) =1664196a(24) =3253531a(25) =6366973a(26) =12471056a(27) =24447507a(28) =47962236a(29) =94161474