2 #include <gsl/gsl_math.h>
3 #include <gsl/gsl_sum.h>
11 double sum_accel, err;
15 gsl_sum_levin_u_workspace * w
16 = gsl_sum_levin_u_alloc (N);
18 const double zeta_2 = M_PI * M_PI / 6.0;
20 /* terms for zeta(2) = \sum_{n=1}^{\infty} 1/n^2 */
22 for (n = 0; n < N; n++)
25 t[n] = 1.0 / (np1 * np1);
29 gsl_sum_levin_u_accel (t, N, w, &sum_accel, &err);
31 printf ("term-by-term sum = % .16f using %d terms\n",
34 printf ("term-by-term sum = % .16f using %d terms\n",
35 w->sum_plain, w->terms_used);
37 printf ("exact value = % .16f\n", zeta_2);
38 printf ("accelerated sum = % .16f using %d terms\n",
39 sum_accel, w->terms_used);
41 printf ("estimated error = % .16f\n", err);
42 printf ("actual error = % .16f\n",
45 gsl_sum_levin_u_free (w);