First let us see that for .
Let be a local basis of
at .
If we set , then
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As
and is surjective,
there are and such that . Therefore,
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so that
, as required.
Next let us see that
for all .
By Proposition 1.9,
is an orthonormal basis of with respect to
. Thus, if
we set (), then
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Finally let us see that for .
For , we choose such that
and
.
Then, by the previous observation,
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Thus the assertion follows.
∎