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Proof.
First we consider the case .
Fix .
For , there is such that and
.
Note that
for all . Let be an -orthogonal basis of with respect to
. If we set
(),
then, by Proposition 1.9,
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so that
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and hence
by taking . Thus the assertion for follows from (1) in Lemma 3.5.
In general, by using (3) in Lemma 3.5,
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and hence we have the assertion by (1) in Lemma 3.5.
∎