nLab
Helmholtz operator

Contents

Idea

The Helmholtz operator (going back to Helmholtz 87, Sonin, with modern version due to Vinogradov 78, Tulczyjew 80) is a map from partial differential equations (on sections of some bundle) to differential operators (on sections of that bundle) with the property that locally its kernel consists precisely of those PDEs which are variational in that there is a local Lagrangian density such that the PDE is the Euler-Lagrange equation which says that the variational derivative of this Lagrangian (equivalently: of the action functional that it induces) vanishes.

In modern terminology this says that together with the Euler-Lagrange operator the Helmholtz operator constitutes a chain complex of abelian sheaves (namely of differential forms on a jet bundle) which is locally exact. In fact this extends in both directions to a locally long exact sequence of forms on the jet bundle, called the Euler-Lagrange complex. See there for more.

Details

A quick way to write the Helmholtz operator HH is as follows: If \mathcal{E} denotes a partial differential equation and LL\mathcal{E} its linearization (evolutionary derivative), and (L *(L\mathcal{E}^\ast its formal adjoint differential operator, as a differential operator then

H[]=L(L) *. H[\mathcal{E}] = L\mathcal{E} - (L\mathcal{E})^\ast \,.

This means that the PDE \mathcal{E} is locally variational precisely if its linearization is formally self-adjoint.

Properties

For proof that every Euler-Lagrange equation is in the kernel of the Helmholtz operator see geometry of physics – A first idea of quantum field theory this prop..

References

The Helmholtz operator originates in

where it is considered for linear ordinary differential equations. The modern general incarnation of the Helmholtz condition is due to

Review includes