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%I M0331 N0126 #36 Jul 08 2023 01:17:54
%S 1,1,2,2,4,8,13,25,44,83,152,286,538,1020,1942,3725,7145,13781,26627,
%T 51572,100099,194633,379037,739250,1443573,2822186,5522889,10818417,
%U 21209278,41613288,81705516,160532194,315604479,620834222,1221918604,2406183020,4740461247
%N Number of positive integers <= 2^n of form x^2 + 16*y^2.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Delbert L. Johnson, <a href="/A000018/b000018.txt">Table of n, a(n) for n = 0..45</a>
%H D. Shanks and L. P. Schmid, <a href="http://dx.doi.org/10.1090/S0025-5718-1966-0210678-1">Variations on a theorem of Landau. Part I</a>, Math. Comp., 20 (1966), 551-569.
%H <a href="/index/Qua#quadpop">Index entries for sequences related to populations of quadratic forms</a>
%o (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))
%K nonn
%O 0,3
%A _N. J. A. Sloane_
%E More terms from _David W. Wilson_, Feb 07 2000
%E Definition corrected by _Sean A. Irvine_, Sep 09 2009
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