DBAR Operator
The DBAR problem, or the \bar-problem, is the problem of solving the differential equation \bar f (z, \bar) = g(z) for the function f(z,\bar), where g(z) is assumed to be known and z = x + iy is a complex number in a domain R\subseteq \Complex. The operator \bar is called the DBAR operator \bar = \frac \left(\frac + i \frac \right) The DBAR operator is nothing other than the complex conjugate of the operator \partial=\frac = \frac \left(\frac - i \frac \right) denoting the usual differentiation in the complex z-plane. The DBAR problem is of key importance in the theory of integrable systems and generalizes the Riemann–Hilbert problem. References {{Reflist DBAR problem The DBAR problem, or the \bar-problem, is the problem of solving the differential equation \bar f (z, \bar) = g(z) for the function f(z,\bar), where g(z) is assumed to be known and z = x + iy is a complex number in a domain R\subseteq \Complex. The ...

Differential Equation
In mathematics, a differential equation is an equation that relates one or more unknown functions and their derivatives. In applications, the functions generally represent physical quantities, the derivatives represent their rates of change, and the differential equation defines a relationship between the two. Such relations are common; therefore, differential equations play a prominent role in many disciplines including engineering, physics, economics, and biology. Mainly the study of differential equations consists of the study of their solutions (the set of functions that satisfy each equation), and of the properties of their solutions. Only the simplest differential equations are solvable by explicit formulas; however, many properties of solutions of a given differential equation may be determined without computing them exactly. Often when a closed-form expression for the solutions is not available, solutions may be approximated numerically using computers. The theory of d ...
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Domain (mathematical Analysis)
In mathematical analysis, a domain or region is a non-empty connected open set in a topological space, in particular any non-empty connected open subset of the real coordinate space or the complex coordinate space . This is a different concept than the domain of a function, though it is often used for that purpose, for example in partial differential equations and Sobolev spaces. The basic idea of a connected subset of a space dates from the 19th century, but precise definitions vary slightly from generation to generation, author to author, and edition to edition, as concepts developed and terms were translated between German, French, and English works. In English, some authors use the term ''domain'', some use the term ''region'', some use both terms interchangeably, and some define the two terms slightly differently; some avoid ambiguity by sticking with a phrase such as ''non-empty connected open subset''. One common convention is to define a ''domain'' as a connected open se ...
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Integrable Systems
In mathematics, integrability is a property of certain dynamical systems. While there are several distinct formal definitions, informally speaking, an integrable system is a dynamical system with sufficiently many conserved quantities, or first integrals, such that its behaviour has far fewer degrees of freedom than the dimensionality of its phase space; that is, its evolution is restricted to a submanifold within its phase space. Three features are often referred to as characterizing integrable systems: * the existence of a ''maximal'' set of conserved quantities (the usual defining property of complete integrability) * the existence of algebraic invariants, having a basis in algebraic geometry (a property known sometimes as algebraic integrability) * the explicit determination of solutions in an explicit functional form (not an intrinsic property, but something often referred to as solvability) Integrable systems may be seen as very different in qualitative character from more ...
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Riemann–Hilbert Problem
In mathematics, Riemann–Hilbert problems, named after Bernhard Riemann and David Hilbert, are a class of problems that arise in the study of differential equations in the complex plane. Several existence theorems for Riemann–Hilbert problems have been produced by Mark Krein, Israel Gohberg and others (see the book by Clancey and Gohberg (1981)). The Riemann problem Suppose that \Sigma is a closed simple contour in the complex plane dividing the plane into two parts denoted by \Sigma_ (the inside) and \Sigma_ (the outside), determined by the index of the contour with respect to a point. The classical problem, considered in Riemann's PhD dissertation (see ), was that of finding a function :M_+(z) = u(z) + i v(z) analytic inside \Sigma_ such that the boundary values of ''M''+ along \Sigma satisfy the equation :a(z)u(z) - b(z)v(z) = c(z) for all z\in \Sigma, where ''a'', ''b'', and ''c'' are given real-valued functions . By the Riemann mapping theorem, it suffices to consider ...
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