docs: public static double pdf(double x, double mu, double shape) {
      if(!(x > 0) || x == Double.POSITIVE_INFINITY) {
        return x == x ? 0 : Double.NaN;
      }
      final double v = (x - mu) / mu;
      double t1 = FastMath.sqrt(shape / (MathUtil.TWOPI * x * x * x));
      return t1 > 0 ? t1 * FastMath.exp(-shape * v * v * .5 / x) : 0;
    }