Main Page
|
See live article
|
Alphabetical index
F-sigma set
In mathematics, an
F-sigma set
(or
F
σ
set)
is a
countable
union
of closed sets. In
metrizable
spaces, every
open set
is an F
σ
set.
The complement of a F
σ
set is a
G
δ
. In a metrizable space, any closed set is a G
δ
set.