2 #include <gsl/gsl_math.h>
3 #include <gsl/gsl_monte.h>
4 #include <gsl/gsl_monte_plain.h>
5 #include <gsl/gsl_monte_miser.h>
6 #include <gsl/gsl_monte_vegas.h>
8 /* Computation of the integral,
10 I = int (dx dy dz)/(2pi)^3 1/(1-cos(x)cos(y)cos(z))
12 over (-pi,-pi,-pi) to (+pi, +pi, +pi). The exact answer
13 is Gamma(1/4)^4/(4 pi^3). This example is taken from
14 C.Itzykson, J.M.Drouffe, "Statistical Field Theory -
15 Volume 1", Section 1.1, p21, which cites the original
16 paper M.L.Glasser, I.J.Zucker, Proc.Natl.Acad.Sci.USA 74
19 /* For simplicity we compute the integral over the region
20 (0,0,0) -> (pi,pi,pi) and multiply by 8 */
22 double exact = 1.3932039296856768591842462603255;
25 g (double *k, size_t dim, void *params)
27 double A = 1.0 / (M_PI * M_PI * M_PI);
28 return A / (1.0 - cos (k[0]) * cos (k[1]) * cos (k[2]));
32 display_results (char *title, double result, double error)
34 printf ("%s ==================\n", title);
35 printf ("result = % .6f\n", result);
36 printf ("sigma = % .6f\n", error);
37 printf ("exact = % .6f\n", exact);
38 printf ("error = % .6f = %.1g sigma\n", result - exact,
39 fabs (result - exact) / error);
47 double xl[3] = { 0, 0, 0 };
48 double xu[3] = { M_PI, M_PI, M_PI };
50 const gsl_rng_type *T;
53 gsl_monte_function G = { &g, 3, 0 };
55 size_t calls = 500000;
60 r = gsl_rng_alloc (T);
63 gsl_monte_plain_state *s = gsl_monte_plain_alloc (3);
64 gsl_monte_plain_integrate (&G, xl, xu, 3, calls, r, s,
66 gsl_monte_plain_free (s);
68 display_results ("plain", res, err);
72 gsl_monte_miser_state *s = gsl_monte_miser_alloc (3);
73 gsl_monte_miser_integrate (&G, xl, xu, 3, calls, r, s,
75 gsl_monte_miser_free (s);
77 display_results ("miser", res, err);
81 gsl_monte_vegas_state *s = gsl_monte_vegas_alloc (3);
83 gsl_monte_vegas_integrate (&G, xl, xu, 3, 10000, r, s,
85 display_results ("vegas warm-up", res, err);
87 printf ("converging...\n");
91 gsl_monte_vegas_integrate (&G, xl, xu, 3, calls/5, r, s,
93 printf ("result = % .6f sigma = % .6f "
94 "chisq/dof = %.1f\n", res, err, s->chisq);
96 while (fabs (s->chisq - 1.0) > 0.5);
98 display_results ("vegas final", res, err);
100 gsl_monte_vegas_free (s);