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 A036698 a(n)=number of Gaussian integers z=a+bi satisfying |z|<=n, a>0, b>=0. 1
 0, 1, 3, 7, 12, 20, 28, 37, 49, 63, 79, 94, 110, 132, 153, 177, 199, 225, 252, 282, 314, 343, 379, 413, 448, 490, 530, 572, 613, 657, 705, 750, 802, 852, 906, 963, 1013, 1073, 1128, 1194, 1256, 1315, 1381, 1447, 1519, 1590, 1656 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS FORMULA a(n) = n^2 * Pi/4 + O(n). - Charles R Greathouse IV, Apr 03 2013 PROG (C) typedef unsigned long ulong; ulong A036698(ulong i) {     const ulong ring = i*i;     ulong result = 0;     for(ulong a = 1; a <= i; a++)     {         const ulong a2 = a*a;         for(ulong b = 0; b <= i; b++)         {             ulong z = a2 + b*b;             if ( ring >= z ) result++;         }     }     return result; } /* Oskar Wieland, Apr 02 2013 */ (PARI) a(n)=my(n2=n^2); sum(a=1, n, sqrtint(n2-a^2)+1) \\ Charles R Greathouse IV, Apr 03 2013 CROSSREFS a(n) = A000603(n) - n - 1. Sequence in context: A002498 A172115 A091369 * A279169 A132273 A130050 Adjacent sequences:  A036695 A036696 A036697 * A036699 A036700 A036701 KEYWORD nonn AUTHOR STATUS approved

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