%I A000074 M1077 N0407
%S A000074 1,1,1,2,4,7,13,25,43,83,157,296,564,1083,2077,4006,7733,14968,29044,
%T A000074 56447,109864,214197,418080,816907,1598040,3129063,6132106,12027122,
%U A000074 23606527,46366165,91127332,179207074,352615528,694182554,1367278759
%N A000074 Number of odd integers <= 2^n of form x^2 + y^2.
%D A000074 D. Shanks, The second-order term in the asymptotic expansion of B(x), 
               Math. Comp., 18 (1964), 75-86.
%D A000074 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A000074 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%H A000074 <a href="Sindx_Qua.html#quadpop">Index entries for sequences related 
               to populations of quadratic forms</a>
%e A000074 a(4)=4 since 2^4=16 and 1=1^2, 5=1^2+2^2, 9=3^2, 13=2^2+3^2.
%o A000074 (PARI) a(n)=if(n<0,0,sum(k=1,2^(n-1),0<sum(y=0,sqrtint(2*k-1),issquare(2*k-1-y^2))))
%Y A000074 Sequence in context: A102112 A102113 A034440 this_sequence A079958 A018082 
               A018083
%Y A000074 Adjacent sequences: A000071 A000072 A000073 this_sequence A000075 A000076 
               A000077
%K A000074 nonn
%O A000074 1,4
%A A000074 N. J. A. Sloane (njas(AT)research.att.com).
%E A000074 9 more terms. Sean A. Irvine (sairvin(AT)xtra.co.nz), Sep 14 2009