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 A000018 Number of positive integers <= 2^n of form x^2 + 16*y^2. (Formerly M0331 N0126) 0
 1, 1, 2, 2, 4, 8, 13, 25, 44, 83, 152, 286, 538, 1020, 1942, 3725, 7145, 13781, 26627, 51572, 100099, 194633, 379037, 739250, 1443573, 2822186, 5522889, 10818417, 21209278, 41613288, 81705516, 160532194, 315604479, 620834222, 1221918604, 2406183020, 4740461247 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS D. Shanks and L. P. Schmid, Variations on a theorem of Landau. Part I, Math. Comp., 20 (1966), 551-569. PROG (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)) CROSSREFS Sequence in context: A039941 A036761 A042979 * A357307 A306604 A354784 Adjacent sequences: A000015 A000016 A000017 * A000019 A000020 A000021 KEYWORD nonn AUTHOR EXTENSIONS More terms from David W. Wilson, Feb 07 2000 Definition corrected by Sean A. Irvine, Sep 09 2009 STATUS approved

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Last modified December 2 05:15 EST 2022. Contains 358485 sequences. (Running on oeis4.)