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Question Number 71551 by mhmd last updated on 17/Oct/19

$$whatsthemean{R}^{++}?and{R}^{--}?$$

Answered by MJS last updated on 17/Oct/19

$$\mathrm{I}\mathrm{only}\mathrm{know}{\mathbb{R}}^{+}=\{x\in \mathbb{R}\mid x>0\}\mathrm{and}$$$${\mathbb{R}}^{-}=\{x\in \mathbb{R}\mid x<0\}$$$$\mathrm{and}$$$$\mathbb{R}\times \mathbb{R}={\mathbb{R}}^2=\{\left(\begin{array}{c}x\\ y\end{array}\right)\mid x\in \mathbb{R}\wedge y\in \mathbb{R}\}$$$$\mathrm{so}\mathrm{maybe}$$$${\mathbb{R}}^{++}={\mathbb{R}}^{+}\times {\mathbb{R}}^{+}=\{\left(\begin{array}{c}x\\ y\end{array}\right)\in {\mathbb{R}}^2\mid x>0\wedge y>0\}$$$$\mathrm{and}\mathrm{similar}\mathrm{for}{\mathbb{R}}^{-+},{\mathbb{R}}^{--},{\mathbb{R}}^{+-}$$

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