src/nmath/stirlerr.c

/*
 *  AUTHOR
 *    Catherine Loader, catherine@research.bell-labs.com.
 *    October 23, 2000.
 *
 *  Merge in to R:
 *	Copyright (C) 2000, The R Core Team
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, a copy is available at
 *  https://www.R-project.org/Licenses/
 *
 *
 *  DESCRIPTION
 *
 *    Computes the log of the error term in Stirling's formula.
 *      For n > 15, uses the series 1/12n - 1/360n^3 + ...
 *      For n <=15, integers or half-integers, uses stored values.
 *      For other n < 15, uses lgamma directly (don't use this to
 *        write lgamma!)
 *
 * Merge in to R:
 * Copyright (C) 2000, The R Core Team
 * R has lgammafn, and lgamma is not part of ISO C
 */

#include "nmath.h"

/* stirlerr(n) = log(n!) - log( sqrt(2*pi*n)*(n/e)^n )
 *             = log Gamma(n+1) - 1/2 * [log(2*pi) + log(n)] - n*[log(n) - 1]
 *             = log Gamma(n+1) - (n + 1/2) * log(n) + n - log(2*pi)/2
 *
 * see also lgammacor() in ./lgammacor.c  which computes almost the same!
 */

double attribute_hidden stirlerr(double n)
{

#define S0 0.083333333333333333333       /* 1/12 */
#define S1 0.00277777777777777777778     /* 1/360 */
#define S2 0.00079365079365079365079365  /* 1/1260 */
#define S3 0.000595238095238095238095238 /* 1/1680 */
#define S4 0.0008417508417508417508417508/* 1/1188 */

/*
  error for 0, 0.5, 1.0, 1.5, ..., 14.5, 15.0.
*/
    const static double sferr_halves[31] = {
	0.0, /* n=0 - wrong, place holder only */
	0.1534264097200273452913848,  /* 0.5 */
	0.0810614667953272582196702,  /* 1.0 */
	0.0548141210519176538961390,  /* 1.5 */
	0.0413406959554092940938221,  /* 2.0 */
	0.03316287351993628748511048, /* 2.5 */
	0.02767792568499833914878929, /* 3.0 */
	0.02374616365629749597132920, /* 3.5 */
	0.02079067210376509311152277, /* 4.0 */
	0.01848845053267318523077934, /* 4.5 */
	0.01664469118982119216319487, /* 5.0 */
	0.01513497322191737887351255, /* 5.5 */
	0.01387612882307074799874573, /* 6.0 */
	0.01281046524292022692424986, /* 6.5 */
	0.01189670994589177009505572, /* 7.0 */
	0.01110455975820691732662991, /* 7.5 */
	0.010411265261972096497478567, /* 8.0 */
	0.009799416126158803298389475, /* 8.5 */
	0.009255462182712732917728637, /* 9.0 */
	0.008768700134139385462952823, /* 9.5 */
	0.008330563433362871256469318, /* 10.0 */
	0.007934114564314020547248100, /* 10.5 */
	0.007573675487951840794972024, /* 11.0 */
	0.007244554301320383179543912, /* 11.5 */
	0.006942840107209529865664152, /* 12.0 */
	0.006665247032707682442354394, /* 12.5 */
	0.006408994188004207068439631, /* 13.0 */
	0.006171712263039457647532867, /* 13.5 */
	0.005951370112758847735624416, /* 14.0 */
	0.005746216513010115682023589, /* 14.5 */
	0.005554733551962801371038690  /* 15.0 */
    };
    double nn;

    if (n <= 15.0) {
	nn = n + n;
	if (nn == (int)nn) return(sferr_halves[(int)nn]);
	return(lgammafn(n + 1.) - (n + 0.5)*log(n) + n - M_LN_SQRT_2PI);
    }

    nn = n*n;
    if (n>500) return((S0-S1/nn)/n);
    if (n> 80) return((S0-(S1-S2/nn)/nn)/n);
    if (n> 35) return((S0-(S1-(S2-S3/nn)/nn)/nn)/n);
    /* 15 < n <= 35 : */
    return((S0-(S1-(S2-(S3-S4/nn)/nn)/nn)/nn)/n);
}