7. Consequences of the Hahn-Banach theorems - Aarhus Universitet
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 />
Pro<strong>of</strong>.<br />
In order to conclude that τ ′ makes X into a topological vector<br />
space it remains now only to show that every one-point set {x} in<br />
X is closed. This goes as follows:<br />
For each y = x <strong>the</strong>re is an element ly ∈ X ′ such that ly(x) = ly(y).<br />
This is because X ′ separates points by assumption. It follows that<br />
{x} c = <br />
{z ∈ X : ly(z) = ly(x)} .<br />
Since<br />
y∈{x} c<br />
{z ∈ X : ly(z) = ly(x)} = l −1<br />
y (Φ\{ly(x)}) ,<br />
we see that {x} c is X ′ -open.<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