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Encyclopedia > Lax pair

In mathematics, in the theory of differential equations, a Lax pair is a pair of time-dependent matrices that describe certain solutions of differential equations. They were developed by Peter Lax to discuss solitons in continuous media. The inverse scattering transform makes use of the Lax equations to solve a variety of the so-called exactly solvable models of physics. Euclid, Greek mathematician, 3rd century BC, known today as the father of geometry; shown here in a detail of The School of Athens by Raphael. ... In mathematics, a differential equation is an equation in which the derivatives of a function appear as variables. ... Peter David Lax (born May 1,1926) is a highly-respected mathematician working in the areas of mathematics. ... In mathematics and physics, a soliton is a self-reinforcing solitary wave caused by nonlinear effects in the medium. ... Continuum mechanics is a branch of physics (specifically mechanics) that deals with continuous matter, including both solids and fluids (i. ... In theoretical physics, an exactly solvable model or integrable model refers to a physical model, a physical theory, or set of differential equations whose exact solution may be calculated analytically in terms of elementary or special functions; the adjective integrable is therefore implies solvablility. ...


Definition

A Lax pair is a pair of matrices L,M such that

frac{dL}{dt}=LM-ML

It can then be shown that the eigenvalues of L are independent of t. The matrices L are said to form an isospectral series. In mathematics, a number is called an eigenvalue of a matrix if there exists a nonzero vector such that the matrix times the vector is equal to the same vector multiplied by the eigenvalue. ... In mathematics, two linear operators are called isospectral if they have the same spectrum. ...


The core observation is that the above equation is the infinitesimal form of a family of matrices L(t) all having the same spectrum, by virtue of being given by

L(t) = A − 1(t)LA(t)

Here, the motion of A can be arbitrarily complicated, yet the solution is still essentially a linear problem.


References

  • P. Lax, Comm. Pure Applied Math. 21 (1968) p. 467
  • P. Lax and R.S. Phillips, Scattering Theory for Automorphic Functions, (1976) Princeton University Press.

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