Gδ set
Countable intersection of open sets / From Wikipedia, the free encyclopedia
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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 from the German nouns Gebiet 'open set' and Durchschnitt 'intersection'.[1] Historically Gδ sets were also called inner limiting sets,[2] but that terminology is not in use anymore. Gδ sets, and their dual, F𝜎 sets, are the second level of the Borel hierarchy.