The principle of stationary action in the calculus of variations
  We review some techniques from non-linear analysis in order to investigate critical paths for the action functional in the calculus of variations applied to physics. Previous attempts to analyse when these are minima ex- ist, but mainly based on physical reasoning and only for a restricted class of models. Our main intention in this regard is to develop precise mathematical conditions for critical paths to be minimum solutions in a variety of situations. Our claim is that, with a few techniques, a systematic analysis (including the domain for which critical points are genuine minima) of non-trivial models is possible. We present specific models arising in modern physical theories in order to make clear the ideas here exposed. 
E. L\'opez, A. Molgado and J. A. Vallejo
math-ph math.MP math.OC