Canonical variational completion of differential equations
  Given a non-variational system of differential equations, the simplest way of turning it into a variational one is by adding a correction term. In the paper, we propose a way of obtaining such a correction term, based on the so-called Vainberg-Tonti Lagrangian, and present several applications in general relativity and classical mechanics. 
Nicoleta Voicu, Demeter Krupka
math-ph math.MP