Number of positive integers <= 2^n of form x^2 + 4 y^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =7a(5) =12a(6) =22a(7) =41a(8) =72a(9) =137a(10) =254a(11) =476a(12) =901a(13) =1716a(14) =3274a(15) =6286a(16) =12090a(17) =23331a(18) =45140a(19) =87511a(20) =169972a(21) =330752a(22) =644499a(23) =1257523a(24) =2456736a(25) =4804666a(26) =9405749a(27) =18429828a(28) =36141339a(29) =70928099