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Oct 28, 2021 at 17:13 comment added Mattis Bakken Thanks so much! All you said is very helpful.
Oct 28, 2021 at 16:40 comment added Leo Moos The answer should indeed be 'yes', at least if $S$ has codimension one. The result should follow from Cauchy-Kovalevskaya theory. Unfortunately I don't know the best reference, but I think Evans covers some of this. Whether there are some issues if $S$ is not embedded or $\dim S \leq \dim M - 2$ I do not know.
Oct 28, 2021 at 16:36 vote accept Mattis Bakken
Oct 28, 2021 at 16:25 comment added Mattis Bakken That's interesting. In my setup, I can actually assume that $u : S \to \mathbf{R}$ is analytic. So you say the answer to my question is 'yes' if $S$ and $u$ are analytic?
Oct 28, 2021 at 16:21 comment added Leo Moos I think the issue would basically be the same: you can't extend non-analytic 'initial data'. That being said---and perhaps you're already aware of this---if the given function $u: S \to \mathbf{R}$ is analytic, then some form of Cauchy-Kovalevskaya should give you the desired harmonic extension.
Oct 28, 2021 at 16:15 comment added Mattis Bakken (+1) Thanks for your answer; that's very interesting. In the examples that I had in mind, the submanifold $S$ was actually real-analytic (in fact holomorphic). Do you think there can still be an issue?
Oct 28, 2021 at 16:08 history edited Leo Moos CC BY-SA 4.0
clarified answer
Oct 28, 2021 at 15:45 history answered Leo Moos CC BY-SA 4.0