Let $P$ be a polynomial with real coefficients. I want to find the necessary&sufficient conditions on $P$ in order for the problem $$u_{tt} + \left[ P ( \partial_x)\right]^2u=0$$


  • $u (0,x) = f(x)$
  • $ u_t(0,x) = 0$

to be well-posed for every positive period of the data $f$.

I am tempted to decompose the PDE to the form $$P_1(\partial_t,\partial_x) \cdot \overline{P_1(\partial_t,\partial_x)}u=0$$ where $P_1(\partial_t,\partial_x) = \partial_t+iP(\partial_x)$ and $\overline{P_1(\partial_t,\partial_x)} =\partial_t - iP(\partial_x) $ is it's complex conjugate. But I don't know how to take it from there...

  • 1
    $\begingroup$ This might be useful. $\endgroup$ Commented May 22, 2018 at 2:52
  • $\begingroup$ @Mattos I saw it, and tried to apply it to here... I'm still trying without success, can you see how to use it? $\endgroup$
    – Uria Mor
    Commented May 23, 2018 at 16:47
  • $\begingroup$ When you say 'well-posed for every positive period of the data $f$,' do you mean that the equation is posed on a periodic domain? (I assume so, since you've tagged the question with [fourier-series]) $\endgroup$
    – user88319
    Commented May 25, 2018 at 14:45
  • $\begingroup$ @Strants Indeed! $\endgroup$
    – Uria Mor
    Commented May 30, 2018 at 20:18


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