public class Test {

  final static double EPS = 1E-8; // tolerance

  static double erf(double x) {
    double temp = 0;
    double term;
    int sign = 1;
    int n = 0;
    double pow = x;
    int fact = 1;
    do {
      term = sign * pow / ((2 * n + 1) * fact);
      temp += term;
      sign *= -1;
      pow = pow * x * x;
      n++;
      fact *= n;
    } while (Math.abs(term) > EPS);

    temp = temp * 2 / Math.sqrt(Math.PI);
    return temp;
  }

  public static void main(String argd[]) {
    double x = 0.5; // test - erf(x) is 0.52050
    double y = erf(x);
    System.out.println(y); // 0.5204998778007922

  }
}