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Gδ set
In the mathematical field of topology, a Gδ set is a subset of a topological space that is a countable intersection of open sets. The notation originated in Germany with ''G'' for ''Gebiet'' (''German'': area, or neighbourhood) meaning open set in this case and δ for ''Durchschnitt'' (''German'': intersection). The term inner limiting set is also used. Gδ sets, and their dual Fσ sets, are the second level of the Borel hierarchy. ==Definition== In a topological space a Gδ set is a countable intersection of open sets. The Gδ sets are exactly the level sets of the Borel hierarchy.
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「Gδ set」の詳細全文を読む
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