docs: public static double pdf(double x, double mu, double sigma) {
      if(x <= 0.) {
        return 0.;
      }
      final double xrel = (FastMath.log(x) - mu) / sigma;
      return 1 / (MathUtil.SQRTTWOPI * sigma * x) * FastMath.exp(-.5 * xrel * xrel);
    }