64 lines
1.5 KiB
C
64 lines
1.5 KiB
C
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#include <stdio.h>
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#include <math.h>
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#include <stdlib.h>
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/* Compute the Euler-Mascheroni constant through the formula
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* of the reciprocal Γ function:
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*
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* 1/Γ(z) = z*e^{γz} pro_{k = 1}^{+ inf} ((1 + z/k) e^{- z/k})
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*
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* Thus:
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*
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* γ = - 1/z * ln(z*Γ(z)*pro{k = 1}^{+ inf} (1 + z/k) e^{- z/k})
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*
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* Product stops when there is no difference between two consecutive
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* terms of the productory due to limited precision.
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*
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* Range of z given in input as min, max and step.
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*
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*/
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double exact = 0.577215664901532860;
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int main(int argc, char** argv) {
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double min = (double)atof(argv[1]);
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double max = (double)atof(argv[2]);
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double step = (double)atof(argv[3]);
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double prev_gamma, gamma = 0;
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double Z;
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double best = 0;
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for (double z = min; z <= max; z += step) {
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prev_gamma = gamma;
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double pro = 1;
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double prev = -1;
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for (double k = 1; pro != prev; k++) {
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prev = pro;
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pro *= (1 + z/k) * exp(- z/k);
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if (z == 9 && pro == prev) printf("k:\t%.0f\n", k);
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}
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double gamma = - 1/z * log(z*tgamma(z)*pro);
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printf("z:\t%.2f\t", z);
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printf("diff:\t%.20f\n", fabs(gamma - exact));
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if ((fabs(gamma - exact) < fabs(prev_gamma - exact)) &&
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(fabs(gamma - exact) < fabs(best - exact))){
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best = gamma;
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Z = z;
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}
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}
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printf("z:\t%.2f\n", Z);
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printf("approx:\t%.20f\n", best);
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printf("true:\t%.20f\n", exact);
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printf("diff:\t%.20f\n", best - exact);
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printf("\t 123456789 123456789 123456789\n");
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return EXIT_SUCCESS;
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}
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