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In mathematics, an Fσ set (said F-sigma set) is a countable union of closed sets. The notation originated in France with F for ''fermé'' (''French'': closed) and σ for ''somme'' (''French'': sum, union).〔.〕 In metrizable spaces, every open set is an Fσ set.〔.〕 The complement of an Fσ set is a Gδ set.〔 In a metrizable space, any closed set is a Gδ set. The union of countably many Fσ sets is an Fσ set, and the intersection of finitely many Fσ sets is an Fσ set. Fσ is the same as in the Borel hierarchy. == Examples == Each closed set is an Fσ set. The set of rationals is an Fσ set. The set of irrationals is not a Fσ set. In a Tychonoff space, each countable set is an Fσ set, because a point is closed. For example, the set of all points in the Cartesian plane such that is rational is an Fσ set because it can be expressed as the union of all the lines passing through the origin with rational slope: : where , is the set of rational numbers, which is a countable set. 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「Fσ set」の詳細全文を読む スポンサード リンク
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