Direct approach to approximate conservation laws

In this paper, non-variational systems of differential equations containing small terms are considered, and a consistent approach for deriving approximate conservation laws through the introduction of approximate Lagrange multipliers is developed. The proposed formulation of the approximate direct method starts by assuming the Lagrange multipliers to be dependent on the small parameter; then, by expanding the dependent variables in power series of the small parameter, we consider the consistent expansion of all the involved quantities (equations and Lagrange multipliers) in such a way the basic principles of perturbation analysis are not violated. Consequently, a theorem leading to the determination of approximate multipliers whence approximate conservation laws arise is proved, and the role of approximate Euler operators emphasized. Some applications of the procedure are presented.