Involutive moving frames

Abstract

By combining the ideas of Cartan's equivalence method and the method of the equivariant moving frame for pseudo-groups, we develop an efficient method for solving equivalence problems arising from horizontal Lie pseudo-group actions. The key is a pseudo-group analog of the classic result that characterizes congruence of submanifolds in Lie groups in terms of equivalence of the Lie group's Maurer-Cartan forms. This result, when combined with the fundamental recurrence formulas for the moving frame for pseudo-groups, will allow for a hybrid equivalence method that will extend and illuminate its two progenitors. Furthermore, incorporating the recurrence formula from the equivariant moving frame calculus provides great computational simplifications in solving equivalence problems. We apply the method to substantial equivalence problems such as (point) equivalence of second order ordinary differential equations, (point) divergence equivalence of Lagrangians on the line and equivalence of linear second order differential operators. These examples will demonstrate the ease with which the combined equivalence method can solve such problems.