[12,∞)\left[\frac{1}{2}, \infty\right)[21,∞)
[12,∞)−{2}\left[\frac{1}{2}, \infty\right)-\{2\}[21,∞)−{2}
(12,∞)−{2}\left(\frac{1}{2}, \infty\right)-\{2\}(21,∞)−{2}
(−∞,∞)(-\infty, \infty)(−∞,∞)
