Divergence-free tensors in the calculus of variations
Résumé
We explain why divergence-free symmetric tensors seem ubiquitous in Mathematical Physics. Several models are described by a varational principle, whose object is a closed differential form. Divergence-free tensors are produced by the so-called "second form" of the Euler-Lagrange equations. Their symmetry seems to be associated with the invariance of the Lagrangian density upon the action of a gauge group.
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