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'''Notation for scalars'''
Suppose that <math>X</math> is a vector space over the [[Field (mathematics)|field]] <math>\mathbb{K}</math> of [[real number]]s <math>\R</math> or [[complex number]]s <math>\Complex,</math> and for any <math>
<math display=block>B_r = \{a \in \mathbb{K} : |a| < r\} \quad \text{ and } \quad B_{\leq r} = \{a \in \mathbb{K} : |a| \leq r\}</math>
denote the ''open ball'' (respectively, the ''closed ball'') of radius <math>r</math> in <math>\mathbb{K}</math> centered at <math>0.</math> <!-- For any <math>-\infty \leq r \leq R \leq \infty</math> and subset <math>A \subseteq X,</math> let
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