In mathematics, the **inverse scattering transform** is a method for solving some non-linear partial differential equations. The method is a non-linear analogue, and in some sense generalization, of the Fourier transform, which itself is applied to solve many linear partial differential equations. The name "inverse scattering method" comes from the key idea of recovering the time evolution of a potential from the time evolution of its scattering data: inverse scattering refers to the problem of recovering a potential from its scattering matrix, as opposed to the direct scattering problem of finding the scattering matrix from the potential.

- Overview
- Example: the Korteweg–de Vries equation
- Method of solution
- Examples of integrable equations
- References
- External links

The inverse scattering transform may be applied to many of the so-called exactly solvable models, that is to say completely integrable infinite dimensional systems.

The inverse scattering transform was first introduced by Clifford S.Gardner,John M. Greene,andMartin D. Kruskalet al. ( 1967 , 1974 ) for the Korteweg–de Vries equation, and soon extended to the nonlinear Schrödinger equation, the Sine-Gordon equation, and the Toda lattice equation. It was later used to solve many other equations, such as the Kadomtsev–Petviashvili equation, the Ishimori equation, the Dym equation, and so on. A further family of examples is provided by the Bogomolny equations (for a given gauge group and oriented Riemannian 3-fold), the solutions of which are magnetic monopoles.

A characteristic of solutions obtained by the inverse scattering method is the existence of solitons, solutions resembling both particles and waves, which have no analogue for linear partial differential equations. The term "soliton" arises from non-linear optics.

The inverse scattering problem can be written as a Riemann–Hilbert factorization problem, at least in the case of equations of one space dimension. This formulation can be generalized to differential operators of order greater than 2 and also to periodic potentials. In higher space dimensions one has instead a "nonlocal" Riemann–Hilbert factorization problem (with convolution instead of multiplication) or a d-bar problem.

The Korteweg–de Vries equation is a nonlinear, dispersive, evolution partial differential equation for a function *u*; of two real variables, one space variable *x* and one time variable *t* :

with and denoting partial derivatives with respect to *t* and *x*, respectively.

To solve the initial value problem for this equation where is a known function of *x*, one associates to this equation the Schrödinger eigenvalue equation

where is an unknown function of *t* and *x* and *u* is the solution of the Korteweg–de Vries equation that is unknown except at . The constant is an eigenvalue.

From the Schrödinger equation we obtain

Substituting this into the Korteweg–de Vries equation and integrating gives the equation

where *C* and *D* are constants.

**Step 1.** Determine the nonlinear partial differential equation. This is usually accomplished by analyzing the physics of the situation being studied.

**Step 2.** Employ *forward scattering*. This consists in finding the Lax pair. The Lax pair consists of two linear operators, and , such that and . It is extremely important that the eigenvalue be independent of time; i.e. Necessary and sufficient conditions for this to occur are determined as follows: take the time derivative of to obtain

Plugging in for yields

Rearranging on the far right term gives us

Thus,

Since , this implies that if and only if

This is Lax's equation. In Lax's equation is that is the time derivative of precisely where it explicitly depends on . The reason for defining the differentiation this way is motivated by the simplest instance of , which is the Schrödinger operator (see Schrödinger equation):

where u is the "potential". Comparing the expression with shows us that thus ignoring the first term.

After concocting the appropriate Lax pair it should be the case that Lax's equation recovers the original nonlinear PDE.

**Step 3.** Determine the time evolution of the eigenfunctions associated to each eigenvalue , the norming constants, and the reflection coefficient, all three comprising the so-called scattering data. This time evolution is given by a system of linear ordinary differential equations which can be solved.

**Step 4.** Perform the *inverse scattering* procedure by solving the Gelfand–Levitan–Marchenko integral equation (Israel Moiseevich Gelfand and Boris Moiseevich Levitan;^{ [1] } Vladimir Aleksandrovich Marchenko ^{ [2] }), a linear integral equation, to obtain the final solution of the original nonlinear PDE. All the scattering data is required in order to do this. If the reflection coefficient is zero, the process becomes much easier. This step works if is a differential or difference operator of order two, but not necessarily for higher orders. In all cases however, the *inverse scattering* problem is reducible to a Riemann–Hilbert factorization problem. (See Ablowitz-Clarkson (1991) for either approach. See Marchenko (1986) for a mathematical rigorous treatment.)

- Korteweg–de Vries equation
- nonlinear Schrödinger equation
- Camassa-Holm equation
- Sine-Gordon equation
- Toda lattice
- Ishimori equation
- Dym equation

Further examples of integrable equations may be found on the article Integrable system.

In mathematics and physics, a **soliton** or **solitary wave** is a self-reinforcing wave packet that maintains its shape while it propagates at a constant velocity. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medium. Solitons are the solutions of a widespread class of weakly nonlinear dispersive partial differential equations describing physical systems.

The **Schrödinger equation** is a linear partial differential equation that governs the wave function of a quantum-mechanical system. It is a key result in quantum mechanics, and its discovery was a significant landmark in the development of the subject. The equation is named after Erwin Schrödinger, who postulated the equation in 1925, and published it in 1926, forming the basis for the work that resulted in his Nobel Prize in Physics in 1933.

In mathematics, the **Korteweg–De Vries (KdV) equation** is a mathematical model of waves on shallow water surfaces. It is particularly notable as the prototypical example of an exactly solvable model, that is, a non-linear partial differential equation whose solutions can be exactly and precisely specified. KdV can be solved by means of the inverse scattering transform. The mathematical theory behind the KdV equation is a topic of active research. The KdV equation was first introduced by Boussinesq and rediscovered by Diederik Korteweg and Gustav de Vries (1895).

In mathematics and physics, the **inverse scattering problem** is the problem of determining characteristics of an object, based on data of how it scatters incoming radiation or particles. It is the inverse problem to the **direct scattering problem**, which is to determine how radiation or particles are scattered based on the properties of the scatterer.

In theoretical physics, the (one-dimensional) **nonlinear Schrödinger equation** (**NLSE**) is a nonlinear variation of the Schrödinger equation. It is a classical field equation whose principal applications are to the propagation of light in nonlinear optical fibers and planar waveguides and to Bose–Einstein condensates confined to highly anisotropic cigar-shaped traps, in the mean-field regime. Additionally, the equation appears in the studies of small-amplitude gravity waves on the surface of deep inviscid (zero-viscosity) water; the Langmuir waves in hot plasmas; the propagation of plane-diffracted wave beams in the focusing regions of the ionosphere; the propagation of Davydov's alpha-helix solitons, which are responsible for energy transport along molecular chains; and many others. More generally, the NLSE appears as one of universal equations that describe the evolution of slowly varying packets of quasi-monochromatic waves in weakly nonlinear media that have dispersion. Unlike the linear Schrödinger equation, the NLSE never describes the time evolution of a quantum state. The 1D NLSE is an example of an integrable model.

The **Compton wavelength** is a quantum mechanical property of a particle. The Compton wavelength of a particle is equal to the wavelength of a photon whose energy is the same as the mass of that particle. It was introduced by Arthur Compton in his explanation of the scattering of photons by electrons.

In physics, a **breather** is a nonlinear wave in which energy concentrates in a localized and oscillatory fashion. This contradicts with the expectations derived from the corresponding linear system for infinitesimal amplitudes, which tends towards an even distribution of initially localized energy.

In mathematics, and in particular in the theory of solitons, the **Dym equation** (**HD**) is the third-order partial differential equation

In mathematics and physics, the **Kadomtsev–Petviashvili equation** is a partial differential equation to describe nonlinear wave motion. Named after Boris Borisovich Kadomtsev and Vladimir Iosifovich Petviashvili, the KP equation is usually written as:

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.

In mathematics, the **KdV hierarchy** is an infinite sequence of partial differential equations which starts with the Korteweg–de Vries equation.

In mathematics and physics, a **nonlinear partial differential equation** is a partial differential equation with nonlinear terms. They describe many different physical systems, ranging from gravitation to fluid dynamics, and have been used in mathematics to solve problems such as the Poincaré conjecture and the Calabi conjecture. They are difficult to study: there are almost no general techniques that work for all such equations, and usually each individual equation has to be studied as a separate problem.

Dispersionless limits of integrable partial differential equations (PDE) arise in various problems of mathematics and physics and have been intensively studied in recent literature. They typically arise when considering slowly modulated long waves of an integrable dispersive PDE system.

In fluid dynamics, a **cnoidal wave** is a nonlinear and exact periodic wave solution of the Korteweg–de Vries equation. These solutions are in terms of the Jacobi elliptic function *cn*, which is why they are coined *cn*oidal waves. They are used to describe surface gravity waves of fairly long wavelength, as compared to the water depth.

In mathematical physics, a **Pöschl–Teller potential**, named after the physicists Herta Pöschl and Edward Teller, is a special class of potentials for which the one-dimensional Schrödinger equation can be solved in terms of special functions.

In mathematics, the **Novikov–Veselov equation** is a natural (2+1)-dimensional analogue of the Korteweg–de Vries (KdV) equation. Unlike another (2+1)-dimensional analogue of KdV, the Kadomtsev–Petviashvili equation, it is integrable via the inverse scattering transform for the 2-dimensional stationary Schrödinger equation. Similarly, the Korteweg–de Vries equation is integrable via the inverse scattering transform for the 1-dimensional Schrödinger equation. The equation is named after S.P. Novikov and A.P. Veselov who published it in Novikov & Veselov (1984).

The **Belinski–Zakharov (inverse) transform** is a nonlinear transformation that generates new exact solutions of the vacuum Einstein's field equation. It was developed by Vladimir Belinski and Vladimir Zakharov in 1978. The Belinski–Zakharov transform is a generalization of the inverse scattering transform. The solutions produced by this transform are called gravitational solitons (gravisolitons). Despite the term 'soliton' being used to describe gravitational solitons, their behavior is very different from other (classical) solitons. In particular, gravitational solitons do not preserve their amplitude and shape in time, and up to June 2012 their general interpretation remains unknown. What is known however, is that most black holes are special cases of gravitational solitons.

In nonlinear systems, the **three-wave equations**, sometimes called the **three-wave resonant interaction equations** or **triad resonances**, describe small-amplitude waves in a variety of non-linear media, including electrical circuits and non-linear optics. They are a set of completely integrable nonlinear partial differential equations. Because they provide the simplest, most direct example of a resonant interaction, have broad applicability in the sciences, and are completely integrable, they have been intensively studied since the 1970s.

**Tau functions** are an important ingredient in the modern theory of integrable systems, and have numerous applications in a variety of other domains. They were originally introduced by **Ryogo Hirota** in his *direct method* approach to soliton equations, based on expressing them in an equivalent bilinear form. The term **Tau function**, or **-function**, was first used systematically by Mikio Sato and his students in the specific context of the Kadomtsev–Petviashvili equation, and related integrable hierarchies. It is a central ingredient in the theory of solitons. Tau functions also appear as matrix model partition functions in the spectral theory of Random Matrices, and may also serve as generating functions, in the sense of combinatorics and enumerative geometry, especially in relation to moduli spaces of Riemann surfaces, and enumeration of branched coverings, or so-called Hurwitz numbers.

The **Schamel equation (S-equation)** is a nonlinear partial differential equation of first order in time and third order in space. Similar to a Korteweg de Vries equation (KdV), it describes the development of a localized, coherent wave structure that propagates in a nonlinear dispersive medium. It was first derived in 1973 by Hans Schamel to describe the effects of electron trapping in the trough of the potential of a solitary electrostatic wave structure travelling with ion acoustic speed in a two-component plasma. It now applies to various localized pulse dynamics such as:

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*Solitons and the Inverse Scattering Transform*, SIAM, Philadelphia, 1981. - N. Asano, Y. Kato,
*Algebraic and Spectral Methods for Nonlinear Wave Equations*, Longman Scientific & Technical, Essex, England, 1990. - M. Ablowitz, P. Clarkson,
*Solitons, Nonlinear Evolution Equations and Inverse Scattering*, Cambridge University Press, Cambridge, 1991. - Gardner, Clifford S.; Greene, John M.; Kruskal, Martin D.; Miura, Robert M. (1967), "Method for Solving the Korteweg-deVries Equation",
*Physical Review Letters*,**19**: 1095–1097, Bibcode:1967PhRvL..19.1095G, doi:10.1103/PhysRevLett.19.1095 - Gardner, Clifford S.; Greene, John M.; Kruskal, Martin D.; Miura, Robert M. (1974), "Korteweg-deVries equation and generalization. VI. Methods for exact solution.",
*Comm. Pure Appl. Math.*,**27**: 97–133, doi:10.1002/cpa.3160270108, MR 0336122 - V. A. Marchenko, "Sturm-Liouville Operators and Applications", Birkhäuser, Basel, 1986.
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*Mathematical Principles of Optical Fiber Communications*, SIAM, Philadelphia, 2004. - Eds: R.K. Bullough, P.J. Caudrey. "Solitons" Topics in Current Physics 17. Springer Verlag, Berlin-Heidelberg-New York, 1980.

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