Baire space

In mathematics, a topological space ${\displaystyle X}$ is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.^[1] According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are examples of Baire spaces. The Baire category theorem combined with the properties of Baire spaces has numerous applications in topology, geometry, and analysis, in particular functional analysis.^[2][3] For more motivation and applications, see the article Baire category theorem. The current article focuses more on characterizations and basic properties of Baire spaces per se.

Bourbaki introduced the term "Baire space"^[4][5] in honor of René Baire, who investigated the Baire category theorem in the context of Euclidean space ${\displaystyle \mathbb {R} ^{n}}$ in his 1899 thesis.^[6]