/**
* The natural logarithm of the beta function.
* @param p require p>0
* @param q require q>0
* @return 0 if p<=0, q<=0 or p+q>2.55E305 to avoid errors and over/underflow
* @author Jaco van Kooten
*/
private static function _logBeta($p, $q)
{
if ($p != self::$_logBetaCache_p || $q != self::$_logBetaCache_q) {
self::$_logBetaCache_p = $p;
self::$_logBetaCache_q = $q;
if ($p <= 0.0 || $q <= 0.0 || $p + $q > LOG_GAMMA_X_MAX_VALUE) {
self::$_logBetaCache_result = 0.0;
} else {
self::$_logBetaCache_result = self::_logGamma($p) + self::_logGamma($q) - self::_logGamma($p + $q);
}
}
return self::$_logBetaCache_result;
}