Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 1 / 22I (Linear Analysis)

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.

If $\Vert Tx\Vert \ge \delta \Vert x\Vert$ and $T{x}_n\to y$, then $x_n$ is Cauchy, so $x_n\to x$ and $y=Tx$; the range is closed. Conversely a closed range is Banach, and $T:X\to T(X)$ has bounded inverse by the Open mapping theorem, giving the lower bound. This is the closed-range criterion for an injection.

If $\Vert Tx\Vert \ge \delta \Vert x\Vert$ and $\Vert S-T\Vert <\delta$, then
$\Vert Sx\Vert \ge (\delta -\Vert S-T\Vert )\Vert x\Vert$. Thus bounded-below maps form an open set.

Surjectivity and Open mapping theorem give $r{B}_Y\subset T(B_X)$. If $\Vert S-T\Vert <r$, iteratively solve with $T$ and correct the error $(S-T)x$; the errors shrink geometrically. Completeness yields an exact preimage under $S$.

For an isomorphism $T$, if $\Vert T^{-1}(S-T)\Vert <1$, thenis invertible by neumann-series perturbation.