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7. Consequences of the Hahn-Banach theorems - Aarhus Universitet

7. Consequences of the Hahn-Banach theorems - Aarhus Universitet

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Weak vector space topologies<br />

Theorem<br />

Theorem 3.10. Let X be a vector space and X ′ a separating vector<br />

space <strong>of</strong> linear functional on X. Then <strong>the</strong> X ′ -topology makes X a<br />

locally convex topological vector space whose dual is X ′ .<br />

The pro<strong>of</strong> is based on <strong>the</strong> following lemma <strong>of</strong> independent interest:<br />

Lemma<br />

Let l1,l2,... ,ln and l be linear functional on <strong>the</strong> vector space X.<br />

The following are equivalent:<br />

(a) There are scalars α1,α2,... ,αn such that<br />

l = α1l1 + α2l2 + · · · + αnln.<br />

Klaus Thomsen <strong>7.</strong> <strong>Consequences</strong> <strong>of</strong> <strong>the</strong> <strong>Hahn</strong>-<strong>Banach</strong> <strong>the</strong>orems

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