 Baire category theorem

The Baire Category Theorem is a fundamental result in functional analysis and topology, particularly in the study of complete metric spaces and topological spaces. It provides insight into the structure of certain types of sets and establishes the notion of "largeness" in the context of topological spaces. The theorem states that in a complete metric space (or, more generally, a Baire space), the intersection of countably many dense open sets is dense.