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Nov 13, 2021 at 18:45 vote accept Riku
Nov 6, 2021 at 16:49 comment added Mateusz Kwaśnicki I would have tried some energy conservation laws, but I never worked with the wave equation really. There are some recent works on this subject, e.g. DOI:10.1515/fca-2018-0067.
Nov 6, 2021 at 13:18 comment added Riku Ok, I see. Then my real question is the following one: without this trick, how can we prove that $$U(t,x) = \sum_{k=1}^\infty \varphi_k(x) \left( \langle u, \varphi_k \rangle \frac{\sin(\sqrt{\lambda_k} t)}{\sqrt{\lambda_k}} \right)$$ (which is the solution of the fractional wave equation in a regular open set $\Omega$ with initial velocity $u$ and zero initial displacement) is smooth in $x \in \Omega$ if $u$ is smooth?
Nov 6, 2021 at 11:08 history answered Mateusz Kwaśnicki CC BY-SA 4.0