Proof of Lemma 3.6 . [015L]
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Proof of Lemma 3.6.
The direct implication follows from the continuity of .
For the reverse implication, assume that
and
consider the following three subsets of :
is the set of functions of the form ,
where ;
is the set of functions of the form
, where ; and
together with the constant function 1.
Then the real vector space spanned
by functions of the form , with is
easily seen to be an -algebra that separates points and contains all
constant functions. By the Stone-Weierstrass Theorem, is
dense in , so it suffices to prove that
for . By linearity,
we may assume with .
We may further assume . Write and
. Then
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which completes the proof.
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