  The Rusk-Skinner formalism was developed in order to give a geometrical
unified formalism for describing mechanical systems. It incorporates all the
characteristics of Lagrangian and Hamiltonian descriptions of these systems
(including dynamical equations and solutions, constraints, Legendre map,
evolution operators, equivalence, etc.).
  In this work we extend this unified framework to first-order classical field
theories, and show how this description comprises the main features of the
Lagrangian and Hamiltonian formalisms, both for the regular and singular cases.
This formulation is a first step toward further applications in optimal control
theory for PDE's.
