Kaup–Kupershmidt equation

The Kaup–Kupershmidt equation (named after David J. Kaup and Boris Abram Kupershmidt) is the nonlinear fifth-order partial differential equation

u_t = u_{xxxxx}%2B10u_{xxx}u%2B25u_{xx}u_x%2B20u^2u_x = \frac16 (6u_{xxxx}%2B60uu_{xx}%2B45u_x^2%2B40u^3)_x.

In addition to it being the first equation in a hierarchy of integrable equations with Lax operator

 \partial_x^3 %2B 2u\partial_x %2B u_x,

it is also the second non-trivial equation in the KdV hierarchy - the first of which is the KdV equation.

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