# Euler operator

*Euler–Lagrange operator*

A fundamental object, , in the calculus of variations (cf. also Variational calculus), used to formulate the system of partial differential equations, called the Euler–Lagrange equations or the variational equations, that the extremals for variational problems must satisfy (cf. also Euler–Lagrange equation).

In essence, to each Lagrangian , the Euler operator assigns a geometric object whose components , , are the expressions for the Euler–Lagrange equations.

For trivial fibre bundles (or locally on appropriate charts) and for first-order Lagrangians, the Euler operator is easy to describe. Thus, suppose , are open sets, is compact, and is the trivial fibre bundle (cf. also Fibre space) over with fibre and projection given by . Here and . Then the first-order jet bundle for is the set

whose points are where . A first-order Lagrangian is a real-valued function that has continuous partial derivatives up to the second order and determines a variational problem as follows.

The set of sections consists of functions of the form

where is twice continuously differentiable. Each section has a -jet , which is the section given by

where . With this notation, the variational problem associated with the Lagrangian is to determine the extreme values of the function , which is the action (or action integral) for :

In the trivial bundle setting, it is an easy exercise to derive the partial differential equations, called the Euler–Lagrange equations (cf. Euler–Lagrange equation), that any extremal of must satisfy. This derivation is given here since it will clarify the difficulties in obtaining the global, or intrinsic, version of these equations when the fibre bundle is not trivial.

For simplicity, assume is a bounded closed interval in and . Suppose that has a maximum or minimum value at . Let be the section

where is any twice continuously differentiable function with compact support in (so, in particular, ). Then for a suitably chosen , the function defined by

has a maximum or minimum value at . Consequently,

In the last equation, denotes the function on the second-order jet bundle defined by

where is the differential operator

In this setting, then, the Euler operator is . The differential operator is called the total derivative operator.

It is important to note that the next to the last equation above comes from integrating by parts and uses the assumption that vanishes on the boundary of .

From the arbitrariness of the variation function (up to the stated conditions), the above shows that must satisfy the second-order partial differential equation

for all . This is the Euler–Lagrange equation for this special case.

For the higher-dimensional cases , (but still first-order Lagrangians), the above variational argument is entirely similar and one can show than each extremal must satisfy the system of partial differential equations

, for all . Here,

and is the differential operator

These expressions involve (Einstein) summation on repeated indices, as is customary (cf. also Einstein rule). Again, the operator is called the total derivative operator and the Euler operator for this setting is the mapping , assigning to each first-order Lagrangian a function on the second-order jet bundle.

Within the trivial bundle setting (or on local charts, for non-trivial bundles), the Euler operator for higher-order Lagrangians , , is also easy to describe. This requires the multi-index notation.

A multi-index is an -tuple of non-negative integers and the order of is . Also,

With this notation, a point in the th-order jet bundle is denoted by

where . For a section , its -jet is given by

where

Using a variational argument similar to that above, but now integrating by parts times, one can show that if the action has a local maximum or minimum value at , then must satisfy the system of partial differential equations

, for all . Here,

where , and is the total derivative operator:

Note that is the multi-index of all zeros except for a in the th position.

In the general setting, the intrinsic construction of the Euler operator is more complicated and many different approaches occur in the literature. See [a1], [a2], [a3], [a4], [a5]. One approach realizes as a certain -form-valued -form on which is globally defined and has, in any chart, the local expression

using Einstein summation, the -s are the local contact -forms, , and is a volume form on the base space . Because of the function in the local expression for the volume form, the components in the local expression of are slightly modified from above to

This approach to the Euler operator is briefly described as follows.

Suppose that is a fibre bundle with -dimensional fibre and base space which is a smooth, -dimensional manifold with volume form . For simplicity of exposition, assume that is compact. The th-order jet bundle consists of equivalence classes of local sections at , all of whose partial derivatives up to order are the same at . There are naturally defined projections and and it is common, to simplify the notation, to identify a differential form on with its pullback to . Thus, and, for a Lagrangian , the action integral can be written as

To make a variation in the action, as was done above in the trivial case, suppose is a vertical vector field on (i.e. for all ) and that is its corresponding flow. Then the prolongation of to a vertical vector field on has flow (cf. also Prolongation of solutions of differential equations). Letting , one has , and consequently

Here, is the Lie derivative of . Suppose now that has compact support contained in the interior of . Use this together with the Stokes theorem to reduce the variation of the action to

The latter equation results from using the variational operator , which maps -forms on into -form-valued contact -forms on . For the case under consideration here, has, on each chart, a component expression:

Consequently, the component expression for the integrand of the first variation is

The problem now is to construct a (horizontal) -form-valued, contact -form on a higher-order jet bundle (as suggested by using integration by parts) so that

and so that , locally on each chart. Consequently, the component expression for the integrand of the first variation is now

Thus, it follows that if the first variation vanishes identically for all of the stated form, then satisfies , which is the global version of the Euler–Lagrange equations. This problem can be solved by using a shift operator .

It is shown in [a1] that there is an operator , called a shift operator, which maps contact-horizontal forms on into -forms on (for ) and which has, on each local chart, the form

By repeated application of in conjunction with and , one gets the Euler operator defined in a global way by

#### References

[a1] | D. Betounes, "Global shift operators and the higher order calculus of variations" J. Geom. Phys. , 10 (1993) pp. 185–201 |

[a2] | I. Kolar, "A geometric version of the higher ordered Hamilton formalism in fibered manifolds" J. Geom. Phys. , 1 (1984) pp. 127–137 |

[a3] | D. Krupka, "Lepagean forms in the higher order variational calculus" , Geometrical Dynamics. Proc. IUTAM–ISIMM Symp. Modern Developments in Analytic. Mech. (Turin, 1982) , I , Technoprint, Bologna (1983) pp. 197–238 |

[a4] | J. Munoz Masque, "Poincare–Cartan forms in higher order variational calculus on fibered manifolds" Rev. Mat. Iberoamercana , 1 (1985) pp. 85–126 |

[a5] | D.J. Sanders, "The geometry of jet bundles" , London Math. Soc. Lecture Notes , 142 , Cambridge Univ. Press (1989) |

**How to Cite This Entry:**

Euler operator. D. Betounes (originator),

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