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:<math>y^{(n)}(x) + \sum_{i=0}^{n-1} a_i(x) y^{(i)}(x) = b(x).</math> (i)
:<math>y^{(n)}(x) + \sum_{i=0}^{n-1} a_i(x) y^{(i)}(x) = b(x).</math> (i)


let <math>y_1(x), \ldots, y_n(x)</math> be a [[Fundamental_system#Homogeneous_equations|fundamental system]] of the corresponding homogeneous equation
let <math>y_1(x), \ldots, y_n(x)</math> be a [[Fundamental_system#Homogeneous_equations|fundamental system]] of solutions of the corresponding homogeneous equation


:<math>y^{(n)}(x) + \sum_{i=0}^{n-1} a_i(x) y^{(i)}(x) = 0.</math> (ii)
:<math>y^{(n)}(x) + \sum_{i=0}^{n-1} a_i(x) y^{(i)}(x) = 0.</math> (ii)

Revision as of 05:18, 10 October 2012

In mathematics, variation of parameters, also known as variation of constants, is a general method to solve inhomogeneous linear ordinary differential equations.

For first-order inhomogeneous linear differential equations it is usually possible to find solutions via integrating factors or undetermined coefficients with considerably less effort, although those methods leverage heuristics that involve guessing and don't work for all inhomogenous linear differential equations.

Variation of parameters extends to linear partial differential equations as well, specifically to inhomogeneous problems for linear evolution equations like the heat equation, wave equation, and vibrating plate equation. In this setting, the method is more often known as Duhamel's principle, named after Jean-Marie Duhamel who first applied the method to solve the inhomogeneous heat equation. Sometimes variation of parameters itself is called Duhamel's principle and vice-versa.

History

The method of variation of parameters was introduced by the Swiss-born mathematician Leonard Euler (1707 - 1783) and completed by the Italian-French mathematician Joseph-Louis Lagrange (1736 - 1813).[1] A forerunner of the method of variation of a celestial body's orbital elements appeared in Euler's work in 1748, while he was studying the mutual perturbations of Jupiter and Saturn.[2] In his 1749 study of the motions of the earth, Euler obtained differential equations for the orbital elements;[3] and in 1753 he applied the method to his study of the motions of the moon.[4] Lagrange first used the method in 1766.[5] Between 1778 and 1783, Lagrange further developed the method both in a series of memoirs on variations in the motions of the planets[6] and in another series of memoirs on determining the orbit of a comet from three observations.[7] (It should be noted that Euler and Lagrange applied this method to nonlinear differential equations and that, instead of varying the coefficients of linear combinations of solutions to homogeneous equations, they varied the constants of the unperturbed motions of the celestial bodies.[8]) During 1808-1810, Lagrange gave the method of variation of parameters its final form in a series of papers.[9]

Description of method

Given an ordinary non-homogeneous linear differential equation of order n

(i)

let be a fundamental system of solutions of the corresponding homogeneous equation

(ii)

Then a particular solution to the non-homogeneous equation is given by

(iii)

where the are continuous functions which satisfy the equations

(iv)

By substituting (iii) into (i) and applying (iv) it follows that

(v)


The linear system (v) of n equations can then be solved using Cramer's rule yielding

where is the Wronskian determinant of the fundamental system and is the Wronskian determinant of the fundamental system with the i-th column replaced by

The particular solution to the non-homogeneous equation can then be written as

Examples

Specific second order equation

Let us solve

We want to find the general solution to the differential equation, that is, we want to find solutions to the homogeneous differential equation

From the characteristic equation

Since we have a repeated root, we have to introduce a factor of x for one solution to ensure linear independence.

So, we obtain u1 = e−2x, and u2 = xe−2x. The Wronskian of these two functions is

Because the Wronskian is non-zero, the two functions are linearly independent, so this is in fact the general solution for the homogeneous differential equation (and not a mere subset of it).

We seek functions A(x) and B(x) so A(x)u1 + B(x)u2 is a general solution of the non-homogeneous equation. We need only calculate the integrals

that is,

where and are constants of integration.

General second order equation

We have a differential equation of the form

and we define the linear operator

where D represents the differential operator. We therefore have to solve the equation for , where and are known.

We must solve first the corresponding homogeneous equation:

by the technique of our choice. Once we've obtained two linearly independent solutions to this homogeneous differential equation (because this ODE is second-order) — call them u1 and u2 — we can proceed with variation of parameters.

Now, we seek the general solution to the differential equation which we assume to be of the form

Here, and are unknown and and are the solutions to the homogeneous equation. Observe that if and are constants, then . We desire A=A(x) and B=B(x) to be of the form

Now,

and since we have required the above condition, then we have

Differentiating again (omitting intermediary steps)

Now we can write the action of L upon uG as

Since u1 and u2 are solutions, then

We have the system of equations

Expanding,

So the above system determines precisely the conditions

We seek A(x) and B(x) from these conditions, so, given

we can solve for (A′(x), B′(x))T, so

where W denotes the Wronskian of u1 and u2. (We know that W is nonzero, from the assumption that u1 and u2 are linearly independent.)

So,

While homogeneous equations are relatively easy to solve, this method allows the calculation of the coefficients of the general solution of the inhomogeneous equation, and thus the complete general solution of the inhomogeneous equation can be determined.

Note that and are each determined only up to an arbitrary additive constant (the constant of integration); one would expect two constants of integration because the original equation was second order. Adding a constant to or does not change the value of because is linear.

References

  1. ^ See:
  2. ^ Euler, L. (1748) "Recherches sur la question des inégalités du mouvement de Saturne et de Jupiter, sujet proposé pour le prix de l'année 1748, par l’Académie Royale des Sciences de Paris" [Investigations on the question of the differences in the movement of Saturn and Jupiter; this subject proposed for the prize of 1748 by the Royal Academy of Sciences (Paris)] (Paris, France: G. Martin, J.B. Coignard, & H.L. Guerin, 1749).
  3. ^ Euler, L. (1749) "Recherches sur la précession des équinoxes, et sur la nutation de l’axe de la terre," Histoire [or Mémoires ] de l'Académie Royale des Sciences et Belles-lettres (Berlin), pages 289-325 [published in 1751].
  4. ^ Euler, L. (1753) Theoria motus lunae: exhibens omnes ejus inaequalitates … [The theory of the motion of the moon: demonstrating all of its inequalities … ] (Saint Petersburg, Russia: Academia Imperialis Scientiarum Petropolitanae [Imperial Academy of Science (St. Petersburg)], 1753).
  5. ^ Lagrange, J.-L. (1766) “Solution de différens problèmes du calcul integral,” Mélanges de philosophie et de mathématique de la Société royale de Turin, vol. 3, pages 179-380.
  6. ^ See:
  7. ^ See:
  8. ^ Michael Efroimsky (2002) "Implicit gauge symmetry emerging in the N-body problem of celestial mechanics," page 3.
  9. ^ See:
    • Lagrange, J.-L. (1808) “Sur la théorie des variations des éléments des planètes et en particulier des variations des grands axes de leurs orbites,” Mémoires de la première Classe de l’Institut de France. Reprinted in: Joseph-Louis Lagrange with Joseph-Alfred Serret, ed., Oeuvres de Lagrange (Paris, France: Gauthier-Villars, 1873), vol. 6, pages 713-768.
    • Lagrange, J.-L. (1809) “Sur la théorie générale de la variation des constantes arbitraires dans tous les problèmes de la méchanique,” Mémoires de la première Classe de l’Institut de France. Reprinted in: Joseph-Louis Lagrange with Joseph-Alfred Serret, ed., Oeuvres de Lagrange (Paris, France: Gauthier-Villars, 1873), vol. 6, pages 771-805.
    • Lagrange, J.-L. (1810) “Second mémoire sur la théorie générale de la variation des constantes arbitraires dans tous les problèmes de la méchanique, … ,” Mémoires de la première Classe de l’Institut de France. Reprinted in: Joseph-Louis Lagrange with Joseph-Alfred Serret, ed., Oeuvres de Lagrange (Paris, France: Gauthier-Villars, 1873), vol. 6, pages 809-816.
  • Coddington, Earl A.; Levinson, Norman (1955). Theory of Ordinary Differential Equations. New York: McGraw-Hill.
  • Boyce, W. E.; DiPrima, R. C. (1965). Elementary Differential Equations and Boundary Value Problems 8th Edition. Wiley Interscience., pages 186-192, 237-241
  • Teschl, Gerald. Ordinary Differential Equations and Dynamical Systems. Providence: American Mathematical Society.