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

It is the first equation in a hierarchy of integrable equations with the Lax operator

 . It has properties similar (but not identical) to those of the better-known KdV hierarchy in which the Lax operator has order 2. References

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Partial differential equations
Integrable systems