The notation originated in French with F for fermé (French: closed) and σ for somme (French: sum, union).
of rationals is an Fσ set in
More generally, any countable set in a T1 space is an Fσ set, because every singleton
of irrationals is not an Fσ set.
[2] The union of countably many Fσ sets is an Fσ set, and the intersection of finitely many Fσ sets is an Fσ set.
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
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