Hyperbolic partial differential equation explained
In mathematics, a hyperbolic partial differential equation of order
is a
partial differential equation (PDE) that, roughly speaking, has a well-posed
initial value problem for the first
derivatives. More precisely, the
Cauchy problem can be locally solved for arbitrary initial data along any non-characteristic
hypersurface. Many of the equations of
mechanics are hyperbolic, and so the study of hyperbolic equations is of substantial contemporary interest. The model hyperbolic equation is the
wave equation. In one spatial dimension, this is
The equation has the property that, if and its first time derivative are arbitrarily specified initial data on the line (with sufficient smoothness properties), then there exists a solution for all time .
The solutions of hyperbolic equations are "wave-like". If a disturbance is made in the initial data of a hyperbolic differential equation, then not every point of space feels the disturbance at once. Relative to a fixed time coordinate, disturbances have a finite propagation speed. They travel along the characteristics of the equation. This feature qualitatively distinguishes hyperbolic equations from elliptic partial differential equations and parabolic partial differential equations. A perturbation of the initial (or boundary) data of an elliptic or parabolic equation is felt at once by essentially all points in the domain.
Although the definition of hyperbolicity is fundamentally a qualitative one, there are precise criteria that depend on the particular kind of differential equation under consideration. There is a well-developed theory for linear differential operators, due to Lars Gårding, in the context of microlocal analysis. Nonlinear differential equations are hyperbolic if their linearizations are hyperbolic in the sense of Gårding. There is a somewhat different theory for first order systems of equations coming from systems of conservation laws.
Definition
A partial differential equation is hyperbolic at a point
provided that the
Cauchy problem is uniquely solvable in a neighborhood of
for any initial data given on a
non-characteristic hypersurface passing through
. Here the prescribed initial data consist of all (transverse) derivatives of the function on the surface up to one less than the order of the differential equation.
Examples
By a linear change of variables, any equation of the formwithcan be transformed to the wave equation, apart from lower order terms which are inessential for the qualitative understanding of the equation. This definition is analogous to the definition of a planar hyperbola.
The one-dimensional wave equation:is an example of a hyperbolic equation. The two-dimensional and three-dimensional wave equations also fall into the category of hyperbolic PDE. This type of second-order hyperbolic partial differential equation may be transformed to a hyperbolic system of first-order differential equations.
Hyperbolic system of partial differential equations
The following is a system of
first order partial differential equations for
unknown
functions where
where
\vec{f}j\inC1(Rs,Rs),j=1,\ldots,d
are once
continuously differentiable functions,
nonlinear in general.
Next, for each
define the
Jacobian matrixThe system is hyperbolic if for all
\alpha1,\ldots,\alphad\inR
the matrix
A:=\alpha1A1+ … +\alphadAd
has only
real eigenvalues and is
diagonalizable.
If the matrix
has
distinct real eigenvalues, it follows that it is diagonalizable. In this case the system is called
strictly hyperbolic.
If the matrix
is symmetric, it follows that it is diagonalizable and the eigenvalues are real. In this case the system is called
symmetric hyperbolic.
Hyperbolic system and conservation laws
There is a connection between a hyperbolic system and a conservation law. Consider a hyperbolic system of one partial differential equation for one unknown function
. Then the system has the form
Here,
can be interpreted as a quantity that moves around according to the
flux given by
. To see that the quantity
is conserved,
integrate over a domain
If
and
are sufficiently smooth functions, we can use the
divergence theorem and change the order of the integration and
to get a conservation law for the quantity
in the general form
which means that the time rate of change of
in the domain
is equal to the net flux of
through its boundary
. Since this is an equality, it can be concluded that
is conserved within
.
See also
Further reading
- A. D. Polyanin, Handbook of Linear Partial Differential Equations for Engineers and Scientists, Chapman & Hall/CRC Press, Boca Raton, 2002.
External links