Conservation laws that depend on functions and PDE reduction: extending Noether $1\tfrac{1}{2}$

Abstract

This paper develops methods for simplifying systems of partial differential equations that have families of conservation laws which depend on functions of the independent or dependent variables. In some cases, such methods can be combined with reduction using families of symmetries, giving a multiple reduction that is analogous to the double reduction of order for ordinary differential equations with variational symmetries. Applications are given, including a widely-used class of pseudoparabolic equations and several mean curvature equations.