The derivative of u with respect to y approaches zero uniformly in x as n increases, but the solution is. The Cauchy problem for the Laplace equation is called ill-posed or not well-posed , since the solution does not continuously depend on the data of the problem. Such ill-posed problems are not usually satisfactory for physical applications. The existence of solutions for the Navier—Stokes equations , a partial differential equation, is part of one of the Millennium Prize Problems. For example, the wave equation described below can be written as.

Some linear, second-order partial differential equations can be classified as parabolic , hyperbolic and elliptic. Others, such as the Euler—Tricomi equation , have different types in different regions. The classification provides a guide to appropriate initial and boundary conditions and to the smoothness of the solutions. This form is analogous to the equation for a conic section:. If there are n independent variables x 1 , x 2 ,… x n , a general linear partial differential equation of second order has the form.

## Recent Topics in Nonlinear PDE, Volume 98 - 1st Edition

The classification depends upon the signature of the eigenvalues of the coefficient matrix a i , j. The partial differential equation takes the form. If a hypersurface S is given in the implicit form. The geometric interpretation of this condition is as follows: if data for u are prescribed on the surface S , then it may be possible to determine the normal derivative of u on S from the differential equation.

If the data on S and the differential equation determine the normal derivative of u on S , then S is non-characteristic.

If the data on S and the differential equation do not determine the normal derivative of u on S , then the surface is characteristic , and the differential equation restricts the data on S : the differential equation is internal to S. If a PDE has coefficients that are not constant, it is possible that it will not belong to any of these categories but rather be of mixed type. A simple but important example is the Euler—Tricomi equation. In the phase space formulation of quantum mechanics, one may consider the quantum Hamilton's equations for trajectories of quantum particles.

These equations are infinite-order PDEs.

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The evolution equation of the Wigner function is also an infinite-order PDE. The quantum trajectories are quantum characteristics , with the use of which one could calculate the evolution of the Wigner function. Linear PDEs can be reduced to systems of ordinary differential equations by the important technique of separation of variables. This technique rests on a characteristic of solutions to differential equations: if one can find any solution that solves the equation and satisfies the boundary conditions, then it is the solution this also applies to ODEs.

We assume as an ansatz that the dependence of a solution on the parameters space and time can be written as a product of terms that each depend on a single parameter, and then see if this can be made to solve the problem. In the method of separation of variables, one reduces a PDE to a PDE in fewer variables, which is an ordinary differential equation if in one variable — these are in turn easier to solve.

## Nonlinear PDEs, Their Geometry, and Applications

This is possible for simple PDEs, which are called separable partial differential equations , and the domain is generally a rectangle a product of intervals. Separable PDEs correspond to diagonal matrices — thinking of "the value for fixed x " as a coordinate, each coordinate can be understood separately. This generalizes to the method of characteristics , and is also used in integral transforms.

In special cases, one can find characteristic curves on which the equation reduces to an ODE — changing coordinates in the domain to straighten these curves allows separation of variables, and is called the method of characteristics. This corresponds to diagonalizing an operator. An important example of this is Fourier analysis , which diagonalizes the heat equation using the eigenbasis of sinusoidal waves. If the domain is finite or periodic, an infinite sum of solutions such as a Fourier series is appropriate, but an integral of solutions such as a Fourier integral is generally required for infinite domains.

The solution for a point source for the heat equation given above is an example of the use of a Fourier integral.

### An International Mathematical Journal

Often a PDE can be reduced to a simpler form with a known solution by a suitable change of variables. Inhomogeneous equations can often be solved for constant coefficient PDEs, always be solved by finding the fundamental solution the solution for a point source , then taking the convolution with the boundary conditions to get the solution.

This is analogous in signal processing to understanding a filter by its impulse response.

The superposition principle applies to any linear system, including linear systems of PDEs. The same principle can be observed in PDEs where the solutions may be real or complex and additive. There are no generally applicable methods to solve nonlinear PDEs. Still, existence and uniqueness results such as the Cauchy—Kowalevski theorem are often possible, as are proofs of important qualitative and quantitative properties of solutions getting these results is a major part of analysis. Nevertheless, some techniques can be used for several types of equations.

The h -principle is the most powerful method to solve underdetermined equations. The Riquier—Janet theory is an effective method for obtaining information about many analytic overdetermined systems. The method of characteristics can be used in some very special cases to solve partial differential equations. In some cases, a PDE can be solved via perturbation analysis in which the solution is considered to be a correction to an equation with a known solution.

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## Recent Topics In Nonlinear PDE IV

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### Table of contents

We have a dedicated site for Germany. The lectures in the first part of this volume were delivered by experts in nonlinear differential equations and their applications to physics. Original research articles from members of the school comprise the second part of this volume. Much of the latter half of the volume complements the methods expounded in the first half by illustrating additional applications of geometric theory of differential equations.