%I A000018 M0331 N0126
%S A000018 1,1,2,2,4,8,13,25,44,83,152,286,538,1020,1942,3725,7145,13781,26627,51572,
%T A000018 100099,194633,379037,739250,1443573,2822186,5522889,10818417,21209278,
               41613288,
%U A000018 81705516,160532194,315604479,620834222,1221918604,2406183020,4740461247
%N A000018 Number of positive integers <= 2^n of form x^2 + 16y^2.
%D A000018 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A000018 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A000018 D. Shanks and L. P. Schmid, Variations on a theorem of Landau. Part I, 
               Math. Comp., 20 (1966), 551-569.
%H A000018 <a href="Sindx_Qua.html#quadpop">Index entries for sequences related 
               to populations of quadratic forms</a>
%o A000018 (PARI) a(n)=local(A);if(n<0,0,A=qfrep([1,0;0,16],2^n);sum(k=1,2^n,A[k]!=0))
%Y A000018 Sequence in context: A104700 A036761 A042979 this_sequence A075126 A098788 
               A153999
%Y A000018 Adjacent sequences: A000015 A000016 A000017 this_sequence A000019 A000020 
               A000021
%K A000018 nonn
%O A000018 0,3
%A A000018 N. J. A. Sloane (njas(AT)research.att.com).
%E A000018 More terms from David W. Wilson (davidwwilson(AT)comcast.net), Feb 07 
               2000.
%E A000018 Definition corrected by Sean A. Irvine (sairvin(AT)xtra.co.nz), Sep 09 
               2009