public static double standardNormalCdf(double x){
  double a=(x) / (Math.sqrt(2.0));
  return 0.5 * (1.0 + ErrorFunction.erf(a));
}

public static double standardNormalCdf ( double x ) { double a = ( x ) / ( Math . sqrt ( 2.0 ) ) ; return 0.5 * ( 1.0 + ErrorFunction . erf ( a ) ) ; }
cumulative density function of the standard normal distribution
