code: public static double logpdf(double x, double mu, double sigma) {
      if(x <= 0.) {
        return Double.NEGATIVE_INFINITY;
      }
      final double xrel = (FastMath.log(x) - mu) / sigma;
      return MathUtil.LOG_ONE_BY_SQRTTWOPI - FastMath.log(sigma * x) - .5 * xrel * xrel;
    }
docs: Probability density function of the normal distribution.
  
  <pre>
  1/(SQRT(2*pi)*sigma*x) * e^(-log(x-mu)^2/2sigma^2)
  </pre>
  
  @param x The value.
  @param mu The mean.
  @param sigma The standard deviation.
  @return logPDF of the given normal distribution at x.