**Solutions of ordinary of differential equation**

Given a differential equation

a function u: I⊂**R → R is called the solution** or integral curve for *F, if u is n-times differentiable on I*, and

Given two solutions u: J⊂R → R and v: I⊂R → R, u is called an extension of v if I⊂J and

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 nconstants 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.

[edit] Examples

Main article: Examples of differential equations

[edit**] 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),

define a new family of unknown functions

for i from 1 to n.

The original differential equation can be rewritten as the system of differential equations with order 1 and dimension n given by

which can be written concisely in vector notation as

with

and

[edit**] Linear ordinary differential equations**

Main article: Linear differential equation

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

which we can write concisely using matrix and vector notation as

with

[edit**] Homogeneous equations**

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

forms an n-dimensional vector space. Given a basis for this vector space , which is called a **fundamental system**, every solution can be written as

The n × n matrix

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.

[edit**] Nonhomogeneous equations**

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

can be constructed by finding the fundamental system to the corresponding homogeneous equation and one particular solution to the inhomogeneous equation. Every solution to nonhomogeneous equation can then be written as

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

Concerning second order linear ordinary differential equations, it is well known that

So, if yh is a solution of: y'' + Py' + Qy = 0 , thensuch that: Q = − s' − s2 − sP.

So, if yh is a solution of: y'' + Py' + Qy = 0; then a particular solution yp of y'' + Py' + Qy = W , is given by:

. [1]

[edit**] Fundamental systems for homogeneous equations with constant coefficients**

If a system of homogeneous linear differential equations has constant coefficients

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

with solution as a matrix exponential

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

and then evaluate the Jordan blocks

of J separately as

[edit**] Theories of ODEs**

[edit**] 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.

[edit**] Reduction to quadratures**

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 nth 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.

[edit**] 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.

[edit] Lie's theory

From 1870 Sophus 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.

A general approach to solve DE's uses the symmetry property of differential equations, the continuous infinitesimal transformations of solutions to solutions (Lie theory). Continuous group theory, Lie algebras and differential geometry are used to understand the structure of linear and nonlinear (partial) differential equations for generating integrable equations, to find its Lax pairs, recursion operators, Bäcklund transform and finally finding exact analytic solutions to the DE.

Symmetry methods have been recognized to study differential equations arising in mathematics, physics, engineering, and many other disciplines.

[edit**] Sturm–Liouville theory**

Sturm–Liouville theory is a theory of eigenvalues and eigenfunctions of linear operators defined in terms of second-order homogeneous linear equations, and is useful in the analysis of certain partial differential equations.