I was reading about Hausdorff paradox and i met this statement "It involves the sphere S2 (a 2-dimensional sphere in R3)" and i don't know what it means or how it appears in a paper P.S I am a novice mathematician. Thank you ye mathematicians..! ;a ;
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$\begingroup$The definition of a $n$-sphere is simply: $$\Bbb S^n = \left\{{x \in \mathbb{R}^{n+1} : \left|{x - y}\right| = r}\right\}$$
So that a two dimensional sphere is simply the 2D surface of an "ordinary" ball in three dimensional space.
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