docs: public static double pdf(double x, double dof) {
      if(x <= 0) {
        return 0.0;
      }
      if(dof <= 0) {
        return Double.NaN;
      }
      final double k = dof * .5;
      return (Math.abs(k - 1.0) < Double.MIN_NORMAL) ? FastMath.exp(-x * 2.0) * 2.0 : //
          FastMath.exp((k - 1.0) * FastMath.log(x * 2.0) - x * 2.0 - logGamma(k)) * 2.0;
    }