docs: public static double pdf(double x, double k, double theta) {
      if(x < 0) {
        return 0.;
      }
      if(x == 0) {
        return (k == 1.) ? theta : 0;
      }
      if(k == 1.) {
        return FastMath.exp(-x * theta) * theta;
      }
      final double xt = x * theta;
      return (xt == Double.POSITIVE_INFINITY) ? 0. : //
          FastMath.exp((k - 1.0) * FastMath.log(xt) - xt - logGamma(k)) * theta;
    }