Ordinary differential equations
An ordinary differential equation (ODE) is an equation containing an unknown function of one real or complex variable x, its derivatives, and some given functions of x. The unknown function is generally represented by a variable (often denoted y), which, therefore, depends on x. Thus x is often called the independent variable of the equation. The term "ordinary" is used in contrast with the term partial differential equation, which may be with respect to more than one independent variable.
Linear differential equations are the differential equations that are linear
in the unknown function and its derivatives. Their theory is well
developed, and in many cases one may express their solutions in terms of
integrals.
Most ODEs that are encountered in physics are linear. Therefore, most special functions may be defined as solutions of linear differential equations (see Holonomic function).
As, in general, the solutions of a differential equation cannot be expressed by a closed-form expression, numerical methods are commonly used for solving differential equations on a computer.
Partial differential equations
A partial differential equation (PDE) is a differential equation that contains unknown multivariable functions and their partial derivatives. (This is in contrast to ordinary differential equations,
which deal with functions of a single variable and their derivatives.)
PDEs are used to formulate problems involving functions of several
variables, and are either solved in closed form, or used to create a
relevant computer model.
PDEs can be used to describe a wide variety of phenomena in nature such as sound, heat, electrostatics, electrodynamics, fluid flow, elasticity, or quantum mechanics.
These seemingly distinct physical phenomena can be formalized similarly
in terms of PDEs. Just as ordinary differential equations often model
one-dimensional dynamical systems, partial differential equations often model multidimensional systems. Stochastic partial differential equations generalize partial differential equations for modeling randomness.
Non-linear differential equations
A non-linear differential equation is a differential equation that is not a linear equation
in the unknown function and its derivatives (the linearity or
non-linearity in the arguments of the function are not considered here).
There are very few methods of solving nonlinear differential equations
exactly; those that are known typically depend on the equation having
particular symmetries. Nonlinear differential equations can exhibit very complicated behaviour over extended time intervals, characteristic of chaos.
Even the fundamental questions of existence, uniqueness, and
extendability of solutions for nonlinear differential equations, and
well-posedness of initial and boundary value problems for nonlinear PDEs
are hard problems and their resolution in special cases is considered
to be a significant advance in the mathematical theory (cf. Navier–Stokes existence and smoothness).
However, if the differential equation is a correctly formulated
representation of a meaningful physical process, then one expects it to
have a solution.[11]
Linear differential equations frequently appear as approximations to nonlinear equations. These approximations are only valid under restricted conditions. For example, the harmonic oscillator equation is an approximation to the nonlinear pendulum equation that is valid for small amplitude oscillations.
Equation order and degree
The order of the differential equation is the highest order of derivative of the unknown function that appears in the differential equation.
For example, an equation containing only first-order derivatives is a first-order differential equation, an equation containing the second-order derivative is a second-order differential equation, and so on.[12][13]
When it is written as a polynomial equation in the unknown function and its derivatives, its degree of the differential equation is, depending on the context, the polynomial degree in the highest derivative of the unknown function,[14] or its total degree in the unknown function and its derivatives. In particular, a linear differential equation has degree one for both meanings, but the non-linear differential equation 
is of degree one for the first meaning but not for the second one.
Differential equations that describe natural phenomena almost
always have only first and second order derivatives in them, but there
are some exceptions, such as the thin-film equation, which is a fourth order partial differential equation.
Examples
In the first group of examples u is an unknown function of x, and c and ω
are constants that are supposed to be known. Two broad classifications
of both ordinary and partial differential equations consist of
distinguishing between linear and nonlinear differential equations, and between homogeneous differential equations and heterogeneous ones.
- Heterogeneous first-order linear constant coefficient ordinary differential equation:
- Homogeneous second-order linear ordinary differential equation:
- Homogeneous second-order linear constant coefficient ordinary differential equation describing the harmonic oscillator:
- Heterogeneous first-order nonlinear ordinary differential equation:
- Second-order nonlinear (due to sine function) ordinary differential equation describing the motion of a pendulum of length L:
In the next group of examples, the unknown function u depends on two variables x and t or x and y.
- Homogeneous first-order linear partial differential equation:
- Homogeneous second-order linear constant coefficient partial differential equation of elliptic type, the Laplace equation:
- Homogeneous third-order non-linear partial differential equation, the KdV equation:
Existence of solutions
Solving differential equations is not like solving algebraic equations.
Not only are their solutions often unclear, but whether solutions are
unique or exist at all are also notable subjects of interest.
For first order initial value problems, the Peano existence theorem gives one set of circumstances in which a solution exists. Given any point 
in the xy-plane, define some rectangular region 
, such that ![{\displaystyle Z=[l,m]\times [n,p]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/515b71ad3b5fc01ff532df8cb71018baca811973)
and is in the interior of . If we are given a differential equation 
and the condition that 
when 
, then there is locally a solution to this problem if 
and 
are both continuous on . This solution exists on some interval with its center at 
. The solution may not be unique. (See Ordinary differential equation for other results.)
However, this only helps us with first order initial value problems. Suppose we had a linear initial value problem of the nth order:
such that
For any nonzero 
, if 
and 
are continuous on some interval containing 
, 
is unique and exists.[15]
Related concepts
Connection to difference equations
The theory of differential equations is closely related to the theory of difference equations,
in which the coordinates assume only discrete values, and the
relationship involves values of the unknown function or functions and
values at nearby coordinates. Many methods to compute numerical
solutions of differential equations or study the properties of
differential equations involve the approximation of the solution of a
differential equation by the solution of a corresponding difference
equation.
Applications
The study of differential equations is a wide field in pure and applied mathematics, physics, and engineering.
All of these disciplines are concerned with the properties of
differential equations of various types. Pure mathematics focuses on the
existence and uniqueness of solutions, while applied mathematics
emphasizes the rigorous justification of the methods for approximating
solutions. Differential equations play an important role in modeling
virtually every physical, technical, or biological process, from
celestial motion, to bridge design, to interactions between neurons.
Differential equations such as those used to solve real-life problems
may not necessarily be directly solvable, i.e. do not have closed form solutions. Instead, solutions can be approximated using numerical methods.
Many fundamental laws of physics and chemistry can be formulated as differential equations. In biology and economics, differential equations are used to model
the behavior of complex systems. The mathematical theory of
differential equations first developed together with the sciences where
the equations had originated and where the results found application.
However, diverse problems, sometimes originating in quite distinct
scientific fields, may give rise to identical differential equations.
Whenever this happens, mathematical theory behind the equations can be
viewed as a unifying principle behind diverse phenomena. As an example,
consider the propagation of light and sound in the atmosphere, and of
waves on the surface of a pond. All of them may be described by the same
second-order partial differential equation, the wave equation,
which allows us to think of light and sound as forms of waves, much
like familiar waves in the water. Conduction of heat, the theory of
which was developed by Joseph Fourier, is governed by another second-order partial differential equation, the heat equation. It turns out that many diffusion processes, while seemingly different, are described by the same equation; the Black–Scholes equation in finance is, for instance, related to the heat equation.
The number of differential equations that have received a name,
in various scientific areas is a witness of the importance of the topic.
See List of named differential equations.
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