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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 describe the corresponding differential equations. They 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. Contents 1 Definition 2 Isospectral property 2.1 Link with the inverse scattering method 3 Example 4 Equations with a Lax pair 5 References Definition A Lax pair is a pair of matrices or operators L(t),P(t) dependent on time and acting on a fixed Hilbert space, and verifying the Lax's equation: where [P,L] = PL − LP is the commutator. Often, as in the example below, P depends on L in a prescribed way, so this is a nonlinear equation for L as a function of t. Isospectral property It can then be shown that the eigenvalues and more generally the spectrum of L are independent of t. The matrices/operators L are said to be isospectral as t varies. The core observation is that the matrices L(t) are all similar by virtue of L(t) = U(t,s)L(s)U(t,s) − 1 where U(t,s) is the solution of the Cauchy problem where I denotes the identity matrix. Note that if L(t) is self-adjoint and P(t) is skew-adjoint, then U(t,s) will be unitary. 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: λ(t) = λ(0) (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 L(0), giving λ and ψ(0,x), In the scattering region where P is known, propagate ψ by using with initial condition ψ(0,x), Knowing ψ in the scattering region, compute L(t) and/or u(t,x). Example The Korteweg–de Vries equation is It can be reformulated as the Lax equation with (a Sturm–Liouville operator) where all derivatives act on all objects to the right. This accounts for the infinite number of first integrals of the KdV equation. 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 References Lax, P. (1968), "Integrals of nonlinear equations of evolution and solitary waves", Comm. Pure Applied Math. 21: 467–490, doi:10.1002/cpa.3160210503  P. Lax and R.S. Phillips, Scattering Theory for Automorphic Functions, (1976) Princeton University Press. This applied mathematics-related article is a stub. You can help Wikipedia by expanding it.v · d · e