Open mapping theorem
The open mapping theorem, also known as the Banach-Schauder theorem, is a fundamental result in functional analysis. It states: if A : X → Y is a surjective continuous linear operator between Banach spaces X and Y, and U is an open set in X, then A(U) is open in Y.
The proof uses the Baire category theorem.
The open mapping theorem has two important consequences:
- If A : X → Y is a bijective continuous linear operator between the Banach spaces X and Y, then the inverse operator A-1 : Y → X is continuous as well.
- If A : X → Y is a linear operator between the Banach spaces X and Y, and if for every sequence (xn) in X with xn → 0 and Axn → y it follows that y = 0, then A is continuous (Closed graph theorem).