Definition:Sphere/Normed Vector Space/Center
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Definition
Let $\struct{X, \norm{\,\cdot\,}}$ be a normed vector space.
Let $x \in X$.
Let $\epsilon \in \R_{>0}$ be a strictly positive real number.
Let $\map {S_\epsilon} x$ be the $\epsilon$-sphere of $x$.
In $\map {S_\epsilon} x$, the value $x$ is referred to as the center of the $\epsilon$-sphere.
Linguistic Note
The British English spelling of center is centre.
The convention on $\mathsf{Pr} \infty \mathsf{fWiki}$ is to use the American English spelling center, but it is appreciated that there may be lapses.