Definitions
Nearby Words

# partial differential equation

In mathematics, an equation that contains partial derivatives, expressing a process of change that depends on more than one independent variable. It can be read as a statement about how a process evolves without specifying the formula defining the process. Given the initial state of the process (such as its size at time zero) and a description of how it is changing (i.e., the partial differential equation), its defining formula can be found by various methods, most based on integration. Important partial differential equations include the heat equation, the wave equation, and Laplace's equation, which are central to mathematical physics.

Equation containing derivatives of a function of a single variable. Its order is the order of the highest derivative it contains (e.g., a first-order differential equation involves only the first derivative of the function). Because the derivative is a rate of change, such an equation states how a function changes but does not specify the function itself. Given sufficient initial conditions, however, such as a specific function value, the function can be found by various methods, most based on integration.

Mathematical statement that contains one or more derivatives. It states a relationship involving the rates of change of continuously changing quantities modeled by functions. Differential equations are very common in physics, engineering, and all fields involving quantitative study of change. They are used whenever a rate of change is known but the process giving rise to it is not. The solution of a differential equation is generally a function whose derivatives satisfy the equation. Differential equations are classified into several broad categories. The most important are ordinary differential equations (ODEs), in which change depends on a single variable, and partial differential equations (PDEs), in which change depends on several variables. Seealso differentiation.

In mathematics, an ordinary differential equation (or ODE) is a relation that contains functions of only one independent variable, and one or more of its derivatives with respect to that variable.

A simple example is Newton's second law of motion, which leads to the differential equation

$m frac\left\{d^2 x\left(t\right)\right\}\left\{dt^2\right\} = F\left(x\left(t\right)\right),,$

for the motion of a particle of mass m. In general, the force F depends upon the position of the particle x(t) at time t, and thus the unknown function x(t) appears on both sides of the differential equation, as is indicated in the notation F(x(t)).

Ordinary differential equations are distinguished from partial differential equations, which involve partial derivatives of several variables.

Ordinary differential equations arise in many different contexts including geometry, mechanics, astronomy and population modelling. Many famous mathematicians have studied differential equations and contributed to the field, including Newton, Leibniz, the Bernoulli family, Riccati, Clairaut, d'Alembert and Euler.

Much study has been devoted to the solution of ordinary differential equations. In the case where the equation is linear, it can be solved by analytical methods. Unfortunately, most of the interesting differential equations are non-linear and, with a few exceptions, cannot be solved exactly. Approximate solutions are arrived at using computer approximations (see numerical ordinary differential equations).

## Definitions

### Ordinary differential equation

Let y be an unknown function

$y: mathbb\left\{R\right\} to mathbb\left\{R\right\}$

in x with $y^\left\{\left(n\right)\right\}$ the nth derivative of y, then an equation of the form

$F\left(x,y,y\text{'}, dots, y^\left\{\left(n-1\right)\right\}\right)=y^\left\{\left(n\right)\right\}$

is called an ordinary differential equation (ODE) of order n; for vector valued functions,

$y: mathbb\left\{R\right\} to mathbb\left\{R\right\}^m$,
it is called a system of ordinary differential equations of dimension m.

When a differential equation of order n has the form

$Fleft\left(x, y, y\text{'}, y, dots, y^\left\{\left(n\right)\right\}right\right) = 0$
it is called an implicit differential equation whereas the form
$Fleft\left(x, y, y\text{'}, y, dots, y^\left\{\left(n-1\right)\right\}right\right) = y^\left\{\left(n\right)\right\}$
is called an explicit differential equation.

A differential equation not depending on x is called autonomous.

A differential equation is said to be linear if F can be written as a linear combination of the derivatives of y

$y^\left\{\left(n\right)\right\} = sum_\left\{i=0\right\}^\left\{n-1\right\} a_i\left(x\right) y^\left\{\left(i\right)\right\} + r\left(x\right)$

with ai(x) and r(x) continuous functions in x. The function r(x) is called the source term; if r(x)=0 then the linear differential equation is called homogeneous, otherwise it is called non-homogeneous or inhomogeneous.

### Solutions

Given a differential equation

$F\left(x,y,y\text{'}, dots, y^\left\{\left(n\right)\right\}\right)=0$

a function

$u: I subset mathbb\left\{R\right\} to mathbb\left\{R\right\}$

is called the solution or integral curve for F, if u is n-times differentiable on I, F is defined for all

$\left(x,u,u\text{'}, dots, u^\left\{\left(n\right)\right\}\right) quad x in I$

and

$F\left(x,u,u\text{'}, dots, u^\left\{\left(n\right)\right\}\right)=0 quad x in I.$

Given two solutions

$u: J subset mathbb\left\{R\right\} to mathbb\left\{R\right\}$

and

$v: I subset mathbb\left\{R\right\} to mathbb\left\{R\right\}$

u is called an extension of v if IJ and

$u\left(x\right) = v\left(x\right) quad x in I.,$

A solution which has no extension is called a global solution.

A general solution of an n-th order equation is a solution containing n arbitrary variables, corresponding to n constants of integration. A particular solution is derived from the general solution by setting the constants to particular values, often chosen to fulfill set 'initial conditions or boundary conditions'. A singular solution is a solution that can't be derived from the general solution.

## Reduction to a first order system

Any differential equation of order n can be written as a system of n first-order differential equations. Given an explicit ordinary differential equation of order n and dimension 1,

$Fleft\left(x, y, y\text{'}, y, dots, y^\left\{\left(n-1\right)\right\}right\right) = y^\left\{\left(n\right)\right\}$

we define a new family of unknown functions

$y_n := y^\left\{\left(n-1\right)\right\}.!$

We can then rewrite the original differential equation as a system of differential equations with order 1 and dimension n.

$y_1^\text{'} = y_2$

$vdots$

$y_n^\text{'} = F\left(y_n, dots, y_1, x\right).$

which can be written concisely in vector notation as

$mathbf\left\{y\right\}^\text{'}=mathbf\left\{F\right\}\left(mathbf\left\{y\right\}, x\right)$

with

$mathbf\left\{y\right\}:=\left(y,ldots,y_n\right).$

## Linear ordinary differential equations

A well understood particular class of differential equations is linear differential equations. We can always reduce an explicit linear differential equation of any order to a system of differential equation of order 1

$y_i\text{'}\left(x\right) = sum_\left\{j=1\right\}^n a_\left\{i,j\right\}\left(x\right) y_j + b_i\left(x\right) , mathrm\left\{,\right\} quad i = 1,ldots,n$

which we can write concisely using matrix and vector notation as

$mathbf\left\{y\right\}^\text{'}\left(x\right) = mathbf\left\{A\right\}\left(x\right) mathbf\left\{y\right\}\left(x\right) + mathbf\left\{b\right\}\left(x\right)$

with

$mathbf\left\{y\right\}\left(x\right):=\left(y_1\left(x\right),ldots,y_n\left(x\right)\right)$
$mathbf\left\{b\right\}\left(x\right):=\left(b_1\left(x\right),ldots,b_n\left(x\right)\right)$
$mathbf\left\{A\right\}\left(x\right):=\left(a_\left\{i,j\right\}\left(x\right)\right) , mathrm\left\{,\right\} quad i,j = 1,ldots,n.$

### Homogeneous equations

The set of solutions for a system of homogeneous linear differential equations of order 1 and dimension n

$mathbf\left\{y\right\}^\text{'}\left(x\right) = mathbf\left\{A\right\}\left(x\right) mathbf\left\{y\right\}\left(x\right)$

forms an n-dimensional vector space. Given a basis for this vector space $mathbf\left\{z\right\}_1\left(x\right), ldots, mathbf\left\{z\right\}_n\left(x\right)$, which is called a fundamental system, every solution $mathbf\left\{s\right\}\left(x\right)$ can be written as

$mathbf\left\{s\right\}\left(x\right) = sum_\left\{i=1\right\}^\left\{n\right\} c_i mathbf\left\{z\right\}_i\left(x\right).$

The n × n matrix

$mathbf\left\{Z\right\}\left(x\right) := \left(mathbf\left\{z\right\}_1\left(x\right), ldots, mathbf\left\{z\right\}_n\left(x\right)\right)$

is called fundamental matrix. In general there is no method to explicitly construct a fundamental system, but if one solution is known d'Alembert reduction can be used to reduce the dimension of the differential equation by one.

### Nonhomogeneous equations

The set of solutions for a system of inhomogeneous linear differential equations of order 1 and dimension n

$mathbf\left\{y\right\}^\text{'}\left(x\right) = mathbf\left\{A\right\}\left(x\right) mathbf\left\{y\right\}\left(x\right) + mathbf\left\{b\right\}\left(x\right)$

can be constructed by finding the fundamental system $mathbf\left\{z\right\}_1\left(x\right), ldots, mathbf\left\{z\right\}_n\left(x\right)$ to the corresponding homogeneous equation and one particular solution $mathbf\left\{p\right\}\left(x\right)$ to the inhomogeneous equation. Every solution $mathbf\left\{s\right\}\left(x\right)$ to nonhomogeneous equation can then be written as

$mathbf\left\{s\right\}\left(x\right) = sum_\left\{i=1\right\}^\left\{n\right\} c_i mathbf\left\{z\right\}_i\left(x\right) + mathbf\left\{p\right\}\left(x\right).$

A particular solution to the nonhomogeneous equation can be found by the method of undetermined coefficients or the method of variation of parameters.

### Fundamental systems for homogeneous equations with constant coefficients

If a system of homogeneous linear differential equations has constant coefficients

$mathbf\left\{y\right\}^\text{'}\left(x\right) = mathbf\left\{A\right\} mathbf\left\{y\right\}\left(x\right)$

then we can explicitly construct a fundamental system. The fundamental system can be written as a matrix differential equation

$mathbf\left\{Y\right\}^\text{'} = mathbf\left\{A\right\} mathbf\left\{Y\right\}$

with solution as a matrix exponential

$e^\left\{x mathbf\left\{A\right\}\right\}$

which is a fundamental matrix for the original differential equation. To explicitly calculate this expression we first transform A into Jordan normal form

$e^\left\{x mathbf\left\{A\right\}\right\} = e^\left\{x mathbf\left\{C\right\}^\left\{-1\right\} mathbf\left\{J\right\} mathbf\left\{C\right\}^\left\{1\right\}\right\} = mathbf\left\{C\right\}^\left\{-1\right\} e^\left\{x mathbf\left\{J\right\}\right\} mathbf\left\{C\right\}^\left\{1\right\}$

and then evaluate the Jordan blocks

$J_i =$
begin{bmatrix} lambda_i & 1 & ; & ; ; & ddots & ddots & ; ; & ; & ddots & 1 ; & ; & ; & lambda_i end{bmatrix} of J separately as
$e^\left\{x mathbf\left\{J_i\right\}\right\} = e^\left\{lambda_i x\right\}$
begin{bmatrix} 1 & x & frac{x^2}{2} & dots & frac{x^{n-1}}{(n-1)!} ; & ddots & ddots & ddots & vdots ; & ; & ddots & ddots & frac{x^2}{2} ; & ; & ; & ddots & x ; & ; & ; & ; & 1 end{bmatrix} .

## Theories of ODEs

### Singular solutions

The theory of singular solutions of ordinary and partial differential equations was a subject of research from the time of Leibniz, but only since the middle of the nineteenth century did it receive special attention. A valuable but little-known work on the subject is that of Houtain (1854). Darboux (starting in 1873) was a leader in the theory, and in the geometric interpretation of these solutions he opened a field which was worked by various writers, notably Casorati and Cayley. To the latter is due (1872) the theory of singular solutions of differential equations of the first order as accepted circa 1900.

The primitive attempt in dealing with differential equations had in view a reduction to quadratures. As it had been the hope of eighteenth-century algebraists to find a method for solving the general equation of the $n$th degree, so it was the hope of analysts to find a general method for integrating any differential equation. Gauss (1799) showed, however, that the differential equation meets its limitations very soon unless complex numbers are introduced. Hence analysts began to substitute the study of functions, thus opening a new and fertile field. Cauchy was the first to appreciate the importance of this view. Thereafter the real question was to be, not whether a solution is possible by means of known functions or their integrals, but whether a given differential equation suffices for the definition of a function of the independent variable or variables, and if so, what are the characteristic properties of this function.

### Fuchsian theory

Two memoirs by Fuchs (Crelle, 1866, 1868), inspired a novel approach, subsequently elaborated by Thomé and Frobenius. Collet was a prominent contributor beginning in 1869, although his method for integrating a non-linear system was communicated to Bertrand in 1868. Clebsch (1873) attacked the theory along lines parallel to those followed in his theory of Abelian integrals. As the latter can be classified according to the properties of the fundamental curve which remains unchanged under a rational transformation, so Clebsch proposed to classify the transcendent functions defined by the differential equations according to the invariant properties of the corresponding surfaces f = 0 under rational one-to-one transformations.

### Lie's theory

From 1870 Lie's work put the theory of differential equations on a more satisfactory foundation. He showed that the integration theories of the older mathematicians can, by the introduction of what are now called Lie groups, be referred to a common source; and that ordinary differential equations which admit the same infinitesimal transformations present comparable difficulties of integration. He also emphasized the subject of transformations of contact.

### Sturm-Liouville theory

Sturm-Liouville theory is a general method for resolution of second order linear equations with variable coefficients.

## Bibliography

• A. D. Polyanin and V. F. Zaitsev, Handbook of Exact Solutions for Ordinary Differential Equations (2nd edition)", Chapman & Hall/CRC Press, Boca Raton, 2003. ISBN 1-58488-297-2
• A. D. Polyanin, V. F. Zaitsev, and A. Moussiaux, Handbook of First Order Partial Differential Equations'', Taylor & Francis, London, 2002. ISBN 0-415-27267-X
• D. Zwillinger, Handbook of Differential Equations (3rd edition), Academic Press, Boston, 1997.
• Hartman, Philip, Ordinary Differential Equations, 2nd Ed., Society for Industrial & Applied Math, 2002. ISBN 0-89871-510-5.
• W. Johnson, A Treatise on Ordinary and Partial Differential Equations, John Wiley and Sons, 1913, in University of Michigan Historical Math Collection
• E.L. Ince, Ordinary Differential Equations, Dover Publications, 1958, ISBN 0486603490
• Witold Hurewicz, Lectures on Ordinary Differential Equations, Dover Publications, ISBN 0-486-49510-8