docs: public static double logpdf(double x, double dof) {
      if(x <= 0) {
        return Double.NEGATIVE_INFINITY;
      }
      if(dof <= 0) {
        return Double.NaN;
      }
      final double k = dof * .5;
      final double twox = x * 2.0;
      return twox == Double.POSITIVE_INFINITY ? Double.NEGATIVE_INFINITY : //
          (k - 1.0) * FastMath.log(twox) - twox - logGamma(k) + MathUtil.LOG2;
    }