Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2013/iii/paper-7/1/iv/solution

True. We prove weak-star topology on an entire infinite-dimensional Banach dual is not metrizable. Suppose instead that has a countable local base at zero. Choose a basic weak-star topology neighbourhood , with its conditions involving a finite set . The still form a local base.
For any , the set is a neighbourhood of zero, so some is contained in it. Every functional annihilating , and every scalar multiple of that functional, belongs to . Consequently every such functional also annihilates . This forces
Indeed a finite-dimensional span is norm closed, and the Hahn-Banach theorem supplies a bounded linear functional vanishing on it and nonzero at any point outside it.
It follows that . Each span is a proper finite-dimensional closed vector subspace and has empty interior in the infinite-dimensional Banach space . This contradicts the Baire category theorem. Hence
The uniform norm bound that made the metric work on is absent on the whole dual. The answers to (i), (ii), (iii) and (iv) are therefore all true.

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