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A new conservation theorem. (English) Zbl 1160.35008
A classical result in the calculus of variations is the Noether theorem that every variational symmetry has an associated conservation law. The author proposes an extension of this result to arbitrary differential equations not necessarily of a variational nature. It is based on a novel concept of adjoint for nonlinear equations. The author shows first that the system consisting of the original equation plus its adjoint is variational and then that every symmetry of the original equation can be extended to one of the combined system. Now a straightforward application of the classical Noether theorem yields a conservation law. As the definition of a nonlinear adjoint requires the introduction of additional unknown functions, one obtains in general even an infinite familiy of conservation laws parametrised by solutions of the adjoint equation. As concrete examples the heat and the Korteweg-de Vries equations are studied.

##### MSC:
 35A30 Geometric theory, characteristics, transformations in context of PDEs 58J70 Invariance and symmetry properties for PDEs on manifolds
##### Keywords:
nonlinear adjoint; Lagrangian; conservation law; Noether theorem
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##### References:
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