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## Summary

In mathematics, a Lagrangian system is a pair (Y, L), consisting of a smooth fiber bundle YX and a Lagrangian density L, which yields the Euler–Lagrange differential operator acting on sections of YX.

In classical mechanics, many dynamical systems are Lagrangian systems. The configuration space of such a Lagrangian system is a fiber bundle Q → ℝ over the time axis . In particular, Q = ℝ × M if a reference frame is fixed. In classical field theory, all field systems are the Lagrangian ones.

## Lagrangians and Euler–Lagrange operators

A Lagrangian density L (or, simply, a Lagrangian) of order r is defined as an n-form, n = dim X, on the r-order jet manifold JrY of Y.

A Lagrangian L can be introduced as an element of the variational bicomplex of the differential graded algebra O(Y) of exterior forms on jet manifolds of YX. The coboundary operator of this bicomplex contains the variational operator δ which, acting on L, defines the associated Euler–Lagrange operator δL.

### In coordinates

Given bundle coordinates xλ, yi on a fiber bundle Y and the adapted coordinates xλ, yi, yiΛ, (Λ = (λ1, ...,λk), |Λ| = kr) on jet manifolds JrY, a Lagrangian L and its Euler–Lagrange operator read

$L={\mathcal {L}}(x^{\lambda },y^{i},y_{\Lambda }^{i})\,d^{n}x,$
$\delta L=\delta _{i}{\mathcal {L}}\,dy^{i}\wedge d^{n}x,\qquad \delta _{i}{\mathcal {L}}=\partial _{i}{\mathcal {L}}+\sum _{|\Lambda |}(-1)^{|\Lambda |}\,d_{\Lambda }\,\partial _{i}^{\Lambda }{\mathcal {L}},$

where

$d_{\Lambda }=d_{\lambda _{1}}\cdots d_{\lambda _{k}},\qquad d_{\lambda }=\partial _{\lambda }+y_{\lambda }^{i}\partial _{i}+\cdots ,$

denote the total derivatives.

For instance, a first-order Lagrangian and its second-order Euler–Lagrange operator take the form

$L={\mathcal {L}}(x^{\lambda },y^{i},y_{\lambda }^{i})\,d^{n}x,\qquad \delta _{i}L=\partial _{i}{\mathcal {L}}-d_{\lambda }\partial _{i}^{\lambda }{\mathcal {L}}.$

### Euler–Lagrange equations

The kernel of an Euler–Lagrange operator provides the Euler–Lagrange equations δL = 0.

## Cohomology and Noether's theorems

Cohomology of the variational bicomplex leads to the so-called variational formula

$dL=\delta L+d_{H}\Theta _{L},$

where

$d_{H}\Theta _{L}=dx^{\lambda }\wedge d_{\lambda }\phi ,\qquad \phi \in O_{\infty }^{*}(Y)$

is the total differential and θL is a Lepage equivalent of L. Noether's first theorem and Noether's second theorem are corollaries of this variational formula.