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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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<strong>Consequences</strong> <strong>of</strong> <strong>the</strong> <strong>Hahn</strong>-<strong>Banach</strong> <strong>the</strong>orems<br />

Pro<strong>of</strong>.<br />

By Theorem 3.5 <strong>the</strong>re is a l ∈ X ∗ such that l(M0) = {0} and<br />

l(x0) = 1.<br />

Let x ∈ M. Then x − f (x)x0 ∈ M0 (since<br />

f (−f (x)x0) = f (x) − f (x)f (x0) = 0), and hence<br />

l(x − f (x)x0) = 0.<br />

It follows that l(x) = f (x).<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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