Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 1 / 22I / a / Solution

The open mapping lemma says: if a bounded linear map between Banach spaces satisfies $\overline{T(B_X)}\supset r{B}_Y$, then $T(B_X)$ contains a (possibly smaller) ball. Given $y$, repeatedly choose corrections $x_n$ whose residuals shrink geometrically; completeness makes $\sum x_n$ converge and maps it to $y$. Baire category applied to $Y={\bigcup }_{n}T(n{B}_X)$ supplies the closure hypothesis for a surjective map. This proves the Open mapping theorem.