G-delta set
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In topology, X, a G-delta set (or G_δ set) is a countable intersection of open sets. In metrizable spaces, every closed set is a G_δ set.

The complement of a G_δ set is an F_σ. In a metrizable space, every open set is an F_σ set.

The original article can be found at: http://en.wikipedia.org/wiki/G-delta_set.
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