The core of the proof consists in showing that
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Note that for any of the sections involved in the supremum,
belongs to and satisfies
on .
Therefore .
Conversely fix and . Fix
such that and . Regularizing
(see Appendix) and translating, we can assume
, and
.
Fix . Let be a small ball on which .
We choose so small that the oscillation of is smaller
than on .
Let be a test function with compact support in
and such that in .
We can assume w.l.o.g. that
for some but
for all . This insures that is a smooth
section of for all .
Let be a smooth
positive metric of on (this is possible if
is chosen large enough since is positive).
Let be a positive metric of on which is smooth in
and with Lelong number
(this is again possible if is large
enough, since is ample). Observe that is a smooth
-closed -form with values in (for all ).
Alternatively it is a smooth -closed -form with values
in . Applying Hörmander’s -estimates
(see e.g. [15], chapter VIII)
with weight , we find a smooth section
of such that and
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Note that has support in where
is smooth so that both integrals are finite.
Since , this forces . The second
integral is actually bounded from above by ,
where is independent of , since on
and the oscillation of is smaller than on .
Therefore satisfies
and
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where is independent of . Now in a neighborhood of , so
the mean-value inequality applied to the subharmonic functions
yields for all in ,
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if is so small that is bigger
than the oscillation of on .
Therefore satisfies
and
.
Letting , and
completes the proof of the equality.