Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 4 / 23H / a / Solution

For separable Banach $X$, the closed unit ball of $X^{\ast }$ is weak-star compact. Embed it in the product of compact discs by evaluations on a countable dense subset; diagonal subsequences converge on that subset, boundedness extends the limit to all $X$, and the limit functional remains in the ball. Metrizability of the bounded weak-star topology turns sequential compactness into compactness. Scaling gives the general Banach-Alaoglu theorem.

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