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 />
Conversely, let Λ ∈ X ∗ . Then Λ −1 ({z ∈ Φ : |z| < 1}) is X ′ open<br />
and contains 0.<br />
There are <strong>the</strong>refore pairs (li,Vi),i = 1,2,... ,n, such that<br />
0 ∈ l −1<br />
1 (V1) ∩ l −2<br />
2 (V2) ∩ · · · ∩ l −1<br />
n (Vn) ⊆ U,<br />
where li ∈ X ′ and <strong>the</strong> Vi’s are open in Φ.<br />
Note that 0 ∈ Vi for all i. It follows <strong>the</strong>n that Λ(x) = 0 when<br />
x ∈ ker l1 ∩ ker l2 ∩ · · · ∩ ker ln. Indeed, if li(x) = 0 for all i, we see<br />
that<br />
tx ∈ l −1<br />
1 (V1) ∩ l −2<br />
2 (V2) ∩ · · · ∩ l −1<br />
n (Vn) ⊆ U<br />
for all t ∈ Φ, which implies that |t||Λ(x)| = |Λ(tx)| < 1 for all<br />
t ∈ Φ. This is only possible when Λ(x) = 0.<br />
Now Lemma 3.9 implies that<br />
Λ ∈ Span{l1,l2,...,ln} ⊆ 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