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 A000603 Number of nonnegative solutions to x^2 + y^2 <= n^2. (Formerly M2541 N1004) 15
 1, 3, 6, 11, 17, 26, 35, 45, 58, 73, 90, 106, 123, 146, 168, 193, 216, 243, 271, 302, 335, 365, 402, 437, 473, 516, 557, 600, 642, 687, 736, 782, 835, 886, 941, 999, 1050, 1111, 1167, 1234, 1297, 1357, 1424, 1491, 1564, 1636, 1703, 1778, 1852, 1931, 2012, 2095 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Row sums of triangle A255238. - Wolfdieter Lang, Mar 15 2015 REFERENCES H. Gupta, A Table of Values of N_3(t), Proc. National Institute of Sciences of India, 13 (1947), 35-63. 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 T. D. Noe, Table of n, a(n) for n = 0..1000 FORMULA a(n) = n^2 * Pi/4 + O(n). - Charles R Greathouse IV, Apr 03 2013 a(n) = A001182(n) + 2*n + 1. - R. J. Mathar, Jan 07 2015 a(n) = 2*A026702(n) - (1 + floor(n/sqrt(2))), n >= 0. - Wolfdieter Lang, Mar 15 2015 a(n) = [x^(n^2)] (1 + theta_3(x))^2/(4*(1 - x)), where theta_3() is the Jacobi theta function. - Ilya Gutkovskiy, Apr 15 2018 MATHEMATICA Table[cnt = 0; Do[If[x^2 + y^2 <= n^2, cnt++], {x, 0, n}, {y, 0, n}]; cnt, {n, 0, 51}] (* T. D. Noe, Apr 02 2013 *) Table[If[n==1, 1, 2*Sum[Sum[A255195[[n, n - k + 1]], {k, 1, k}], {k, 1, n}] - Ceiling[(n - 1)/Sqrt[2]]], {n, 1, 52}] (* Mats Granvik, Feb 19 2015 *) PROG (Haskell) a000603 n = length [(x, y) | x <- [0..n], y <- [0..n], x^2 + y^2 <= n^2] -- Reinhard Zumkeller, Jan 23 2012 (PARI) a(n)=my(n2=n^2); sum(a=0, n, sqrtint(n2-a^2)+1) \\ Charles R Greathouse IV, Apr 03 2013 CROSSREFS Column k=2 of A302998. Cf. A000328, A036695, A036702, A255238, A255195. Sequence in context: A281376 A247586 A107957 * A003453 A011901 A169739 Adjacent sequences:  A000600 A000601 A000602 * A000604 A000605 A000606 KEYWORD nonn AUTHOR EXTENSIONS More terms from David W. Wilson, May 22 2000 STATUS approved

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Last modified October 16 13:51 EDT 2019. Contains 328093 sequences. (Running on oeis4.)