# Complete system

closed system (of differential equations)

A system of first-order partial differential equations:

$$\tag{1 } F _ {i} ( x, u , p) = 0 ,\ \ 1 \leq i \leq m,$$

$$x = ( x _ {1} \dots x _ {n} ),\ u = u( x _ {1} \dots x _ {n} ),$$

$$p = ( p _ {1} \dots p _ {n} ) = \left ( \frac{\partial u }{\partial x _ {1} } \dots \frac{\partial u }{\partial x _ {n} } \right ) ,$$

with the following property: For any set of numbers $( x, u , p)$ satisfying (1), the equation

$$F _ {ij} ( x, u , p) = 0,\ \ 1 \leq i, j \leq m,$$

is valid, where $F _ {ij} = [ F _ {i} , F _ {j} ]$ are the Jacobi brackets.

The completeness condition may be formulated somewhat differently for a linear homogeneous system. The Jacobi brackets in that case are linear in the variables $p = ( p _ {1} \dots p _ {n} )$; if the system is written in the form

$$P _ {i} ( u) = 0,\ \ 1 \leq i \leq m,$$

where the $P _ {i}$ are linear first-order differential operators, then these brackets correspond to the commutators $[ P _ {i} , P _ {j} ] = P _ {i} P _ {j} - P _ {j} P _ {i}$. The system is complete if all commutators $[ P _ {i} , P _ {j} ]$ can be expressed as linear combinations of the $P _ {k}$' s with coefficients depending only on $x = ( x _ {1} \dots x _ {n} )$.

If $u = u( x)$ is the joint solution of the two equations

$$F _ {i} ( x, u , p) = 0,\ \ F _ {j} ( x, u , p) = 0,$$

then $u$ is also a solution of the equation

$$\tag{2 } [ F _ {i} , F _ {j} ] ( x, u , p) = 0.$$

An arbitrary system of the form (1) is usually extended to a complete one by adding new independent equations to it that have been obtained from the old ones by means of the formation of Jacobi brackets. In this extension, in accordance with (2), none of the solutions will be lost if the system is generally solvable.

A property of the system is that it is completely invariant under those non-singular transformations of the variables $( x, u , F)$ for which the meaning of the differential equations is retained. These transformations include, for example, diffeomorphisms $g( y)$, $y = ( y _ {1} \dots y _ {n} )$, and also transformations of the following type. Let $H: \mathbf R ^ {2n+} 1+ m \rightarrow \mathbf R ^ {m}$ be a smooth mapping such that

$$y = x,\ q = p,$$

$$v = u ,\ t = H( x, u , p, s),$$

$$s = ( s _ {1} \dots s _ {m} ),\ t = ( t _ {1} \dots t _ {m} ),$$

is a diffeomorphism $\mathbf R ^ {2n+} 1+ m \rightarrow \mathbf R ^ {2n+} 1+ m$. This transforms the system (1) into a system

$$G _ {i} ( x, u , p) = \ H _ {i} ( x, u , p, F ) = 0,\ \ 1 \leq i \leq m.$$

How to Cite This Entry:
Complete system. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Complete_system&oldid=46423
This article was adapted from an original article by A.P. Soldatov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article