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@GHfromMO Well -- I think all three answers are excellent. In particular, you gave the complete references (thanks a lot!), and Aleksei Kulikov (who answered first) gave a very slick and complete proof.
The link in your answer is broken. Would it be possible that you replace it by an up-to-date link -- and also elaborate a bit more, such that this is not just a link-only answer?
