Number of positive integers <= 2^n of form x^2 + 7 y^2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =7a(5) =12a(6) =21a(7) =39a(8) =69a(9) =130a(10) =240a(11) =451a(12) =854a(13) =1620a(14) =3101a(15) =5941a(16) =11443a(17) =22080a(18) =42724a(19) =82835a(20) =160914a(21) =313149a(22) =610230a(23) =1190802a(24) =2326448a(25) =4550161a(26) =8908095a(27) =17455391a(28) =34232368a(29) =67184426