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 A000074 Number of odd integers <= 2^n of form x^2 + y^2. (Formerly M1077 N0407) 1
 1, 1, 2, 4, 7, 13, 25, 43, 83, 157, 296, 564, 1083, 2077, 4006, 7733, 14968, 29044, 56447, 109864, 214197, 418080, 816907, 1598040, 3129063, 6132106, 12027122, 23606527, 46366165, 91127332, 179207074, 352615528, 694182554, 1367278759 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS First differences of A000050. - Jean-François Alcover, Mar 19 2014 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 Daniel Shanks, The second-order term in the asymptotic expansion of B(x), Math. Comp., 18 (1964), 75-86. EXAMPLE a(4)=4 since 2^4=16 and 1=1^2, 5=1^2+2^2, 9=3^2, 13=2^2+3^2. PROG (PARI) a(n)=if(n<0, 0, sum(k=1, 2^(n-1), 0

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