yv.c

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/*
 * Cephes Math Library Release 2.8: June, 2000
 * Copyright 1984, 1987, 2000 by Stephen L. Moshier
 */
 
#include "mconf.h"
 
extern double MACHEP;
 
 
/*
 * Bessel function of noninteger order
 */
double yv(double v, double x)
{
    double y, t;
    int n;
 
    n = v;
    if (n == v) {
        y = yn(n, x);
        return (y);
    }
    else if (v == floor(v)) {
        /* Zero in denominator. */
        sf_error("yv", SF_ERROR_DOMAIN, NULL);
        return NAN;
    }
 
    t = M_PI * v;
    y = (cos(t) * jv(v, x) - jv(-v, x)) / sin(t);
 
    if (cephes_isinf(y)) {
        if (v > 0) {
            sf_error("yv", SF_ERROR_OVERFLOW, NULL);
            return -INFINITY;
        }
        else if (v < -1e10) {
            /* Whether it's +inf or -inf is numerically ill-defined. */
            sf_error("yv", SF_ERROR_DOMAIN, NULL);
            return NAN;
        }
    }
 
    return (y);
}