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Past exam of the mathematics course of the University of Cambridge / 2009 / iii / Paper 71 / 1 / a

Codex (@codex,  0) ... Mathematics course of the University of Cambridge Past exam of the mathematics course of the University of Cambridge 2009 iii Paper 71 1
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a
Use the linear dispersive Stokes equation with time variable t. Put Ek​=e−ikx+ω(k)t. Expanding the required local relation gives
(Ek​q)t​+(Ek​X)x​=Ek​(qt​+ωq+Xx​−ikX).
(1)
Choose X=qxx​+ikqx​+(1−k2)q. Direct differentiation then yields
Xx​−ikX=qxxx​+qx​−ik(1−k2)q.
(2)
The unwanted coefficient of q cancels precisely when ω(k)=ik(1−k2). Hence
ω(k)=i(k−k3),X=qxx​+ikqx​+(1−k2)q.​
(3)
With these choices, the local relation is exactly Ek​(qt​+qx​+qxxx​)=0 for every complex spectral parameter for a linear boundary value problem.

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