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May 14 at 23:49 vote accept David Roberts
May 14 at 11:52 comment added Tobias Diez I like this argument! (For completeness: it is enough to show that the subspaces are closed since the Banach-Schauder theorem applies in the Frechet setting and shows that the decomposition is then a topological isomorphism.)
May 14 at 11:43 comment added David Roberts It is the fact that $d$ and $\delta$ have closed images, which is non-trivial. <--- yes, this was the part that seemed the most mysterious!
May 14 at 11:19 comment added Will Sawin The integration over submanifolds trick is also helpful for understanding variation as the metric varies since it shows that the image of $d$ is fixed as the metric varies and the image of $\delta$ is the translate of a fixed closed subspace by a smoothly varying isomorphism of vector bundles, so both are closed in families.
May 14 at 11:18 comment added Branimir Ćaćić The argument from Poincaré duality is very nice indeed!
May 14 at 9:20 history answered Stefan Waldmann CC BY-SA 4.0