R−{4}\mathbb{R}-\{4\}R−{4}
R−{−12}\mathbb{R}-\left\{-\frac{1}{2}\right\}R−{−21}
R−{−4}\mathbb{R}-\{-4\}R−{−4}
R−{12}\mathbb{R}-\left\{\frac{1}{2}\right\}R−{21}