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Sep 30, 2021 at 19:11 comment added Igor Khavkine @ArnoldNeumaier I've added an update, where I indicate how to handle the operator from your example.
Sep 30, 2021 at 19:08 history edited Igor Khavkine CC BY-SA 4.0
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Sep 29, 2021 at 13:58 comment added Arnold Neumaier I added an example to my question (correcting earlier comments here that were not completely correct).
Sep 29, 2021 at 8:20 comment added Igor Khavkine @ArnoldNeumaier I understand what you mean by this kind of general hyperbolicity condition, but it's still not clear to me what class of systems you are referring to. Maybe you could add an example or an explanation to the original question.
Sep 29, 2021 at 8:06 comment added Arnold Neumaier Independent of the order of the principal symbol, a hyperbolic system can be defined in terms of the microlocal spectrum at each point. It should consist of timelike and antitimelike but not of spacelike directions with respect to some cone.
Sep 28, 2021 at 20:57 comment added Igor Khavkine @ArnoldNeumaier I'm not sure what you mean by "semidefinite". In what sense would such a system be hyperbolic?
Sep 27, 2021 at 20:41 comment added Arnold Neumaier The 2015 paper of Bär is the kind of results I wanted to see. But another case of interest for me is a system of first oder equations where the symbol is only semidefinite in future-directed directions. Are there results generalizing Bär's Theorem 5.8 in this case, with additional assumptions on the differential operator\
Sep 27, 2021 at 20:36 vote accept Arnold Neumaier
Sep 27, 2021 at 13:59 comment added Arnold Neumaier Thanks! I'll study these references to see whether they answer my question.
Sep 27, 2021 at 12:57 history answered Igor Khavkine CC BY-SA 4.0