R,R\mathbb{R}, \mathbb{R}R,R
R−{0},R\mathbb{R}-\{0\}, \mathbb{R}R−{0},R
R+,R−{0}\mathbb{R}^{+}, \mathbb{R}-\{0\}R+,R−{0}
R−{0},{−1,1}\mathbb{R}-\{0\},\{-1,1\}R−{0},{−1,1}