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In mathematics, in the theory of integrable systems, a Lax pair is a pair of time-dependent matrices or operators that satisfy a corresponding differential equation, called the Lax equation. Lax pairs were introduced by Peter Lax to discuss solitons in continuous media. The inverse scattering transform makes use of the Lax equations to solve such systems.
A Lax pair is a pair of matrices or operators dependent on time and acting on a fixed Hilbert space, and satisfying Lax's equation:
where is the commutator. Often, as in the example below, depends on in a prescribed way, so this is a nonlinear equation for as a function of .
The core observation is that the matrices are all similar by virtue of
where is the solution of the Cauchy problem
In other words, to solve the eigenvalue problem Lψ = λψ at time t, it is possible to solve the same problem at time 0 where L is generally known better, and to propagate the solution with the following formulas:
- (no change in spectrum)
Link with the inverse scattering method
The above property is the basis for the inverse scattering method. In this method, L and P act on a functional space (thus ψ = ψ(t,x)), and depend on an unknown function u(t,x) which is to be determined. It is generally assumed that u(0,x) is known, and that P does not depend on u in the scattering region where . The method then takes the following form:
- Compute the spectrum of , giving and ,
- In the scattering region where is known, propagate in time by using with initial condition ,
- Knowing in the scattering region, compute and/or .
Example - KdV
The Korteweg–de Vries equation is
It can be reformulated as the Lax equation
where all derivatives act on all objects to the right. This accounts for the infinite number of first integrals of the KdV equation.
Example - Kowalewski
The previous example used an infinite dimensional Hilbert space. Examples are also possible with finite dimensional Hilbert spaces. These include Kowalewskaja's top and the generalization to include an electric Field . 
Equations with a Lax pair
Further examples of systems of equations that can be formulated as a Lax pair include:
- Benjamin–Ono equation
- One-dimensional cubic non-linear Schrödinger equation
- Davey–Stewartson system
- Kadomtsev–Petviashvili equation
- Korteweg–de Vries equation
- KdV hierarchy
- Modified Korteweg–de Vries equation
- Sine-Gordon equation
- Toda lattice
- Lagrange, Euler, and Kovalevskaya tops
- Lax, P. (1968), "Integrals of nonlinear equations of evolution and solitary waves", Comm. Pure Applied Math., 21 (5): 467–490, doi:10.1002/cpa.3160210503 archive
- P. Lax and R.S. Phillips, Scattering Theory for Automorphic Functions, (1976) Princeton University Press.
- Bobenko, A. I.; Reyman, A. G.; Semenov-Tian-Shansky, M. A. (1989). "The Kowalewski top 99 years later: a Lax pair, generalizations and explicit solutions". Communications in Mathematical Physics. 122 (2): 321–354. ISSN 0010-3616.