double stdtri (double rk, double p)
{
double t, z;
int rflg;
if (rk <= 0 || p <= 0.0 || p >= 1.0) {
mtherr("stdtri", CEPHES_DOMAIN);
return 0.0;
}
if (p > 0.25 && p < 0.75) {
if (p == 0.5) {
return 0.0;
}
z = 1.0 - 2.0 * p;
z = incbi(0.5, 0.5*rk, fabs(z));
t = sqrt(rk * z/(1.0-z));
if (p < 0.5) {
t = -t;
}
return t;
}
rflg = -1;
if (p >= 0.5) {
p = 1.0 - p;
rflg = 1;
}
z = incbi(0.5*rk, 0.5, 2.0*p);
if (MAXNUM * z < rk) {
return rflg * MAXNUM;
}
t = sqrt(rk/z - rk);
return rflg * t;
}
double fdtri(double a, double b, double y)
{
double w, x;
if ((a <= 0.0) || (b <= 0.0) || (y <= 0.0) || (y > 1.0)) {
mtherr("fdtri", DOMAIN);
return CEPHES_NAN;
}
y = 1.0 - y;
/* Compute probability for x = 0.5. */
w = incbet(0.5 * b, 0.5 * a, 0.5);
/* If that is greater than y, then the solution w < .5.
* Otherwise, solve at 1-y to remove cancellation in (b - b*w). */
if (w > y || y < 0.001) {
w = incbi(0.5 * b, 0.5 * a, y);
x = (b - b * w) / (a * w);
}
else {
w = incbi(0.5 * a, 0.5 * b, 1.0 - y);
x = b * w / (a * (1.0 - w));
}
return x;
}
