F-sigma subset

This article defines a property over pairs of a topological space and a subspace, or equivalently, properties over subspace embeddings (viz, subsets) in topological spaces

Definition

Symbol-free definition

A subset of a topological space is termed a $F_{\sigma }$-subset if it satisfies the following equivalent conditions:

• Its complement is a $G_{\delta }$ subset (Further information: G-delta subset
• It is expressible as a countable union of closed subsets of the topological space

Relation with other properties

Stronger properties

• Closed subset

Related properties

• G-delta subset

See F hierarchy and G hierarchy