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Porous set In mathematics, a porous set is a concept in the study of metric spaces. Like the concepts of meagre and measure zero sets, a porous set can be considered "sparse" or "lacking bulk"; however, porous sets are not equivalent to either meagre sets or measure zero sets, as shown below. ==Definition==
Let (''X'', ''d'') be a complete metric space and let ''E'' be a subset of ''X''. Let ''B''(''x'', ''r'') denote the closed ball in (''X'', ''d'') with centre ''x'' ∈ ''X'' and radius ''r'' > 0. ''E'' is said to be porous if there exist constants 0 < ''α'' < 1 and ''r''0 > 0 such that, for every 0 < ''r'' ≤ ''r''0 and every ''x'' ∈ ''X'', there is some point ''y'' ∈ ''X'' with : A subset of ''X'' is called ''σ''-porous if it is a countable union of porous subsets of ''X''.
抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Porous set」の詳細全文を読む
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