Landau's approximation to population of x^2 + y^2 <= 2^n.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =7a(5) =13a(6) =24a(7) =44a(8) =83a(9) =157a(10) =297a(11) =567a(12) =1085a(13) =2086a(14) =4019a(15) =7766a(16) =15039a(17) =29181a(18) =56717a(19) =110408a(20) =215225a(21) =420076a(22) =820836a(23) =1605587a(24) =3143562a(25) =6160098a(26) =12080946a(27) =23710229a(28) =46565965a(29) =91512121