On the exact discrete Lagrangian function for variational integrators:   theory and applications
  In this paper, we will give a rigorous construction of the exact discrete Lagrangian formulation associated to a continuous Lagrangian problem. Moreover, we work in the setting of Lie groupoids and Lie algebroids which is enough general to simultaneously cover several cases of interest in discrete and continuous descriptions as, for instance, Euler-Lagrange equations, Euler-Poincar\'e equations, Lagrange-Poincar\'e equations... The construction of an exact discrete Lagrangian is of considerable interest for the analysis of the error between an exact trajectory and the discrete trajectory derived by a variational integrator. 
J. C. Marrero, D. Mart\'in de Diego and E. Mart\'inez
math.DG math-ph math.MP math.NA