/*
 * Calculate the sum: sum_{k=1}^\infty 1/(4k^2 - 1) =
 *  sum{k=1}^\infty 1/2*(1/(2*k - 1) - 1/(2*k + 1)) = 1/2
 *
 */

#include <stdio.h>
#include <math.h>

#define N 1000000L
#define NREP 10

double simplesum(long n);

int main(void) 
{
    double s;
    int k;
    long n = N;

    for (k = 0; k <= NREP; k++) 
    {
        s = simplesum(n);
        printf("%20ld   %20.14f   %20.14f\n", n, s, fabs(0.5 - s));
        n *= 2;
    }

    return(0);
}