Continuous and Discrete Homotopy Operators: A Theoretical Approach made   Concrete
  Using standard calculus, explicit formulas for the one-dimensional continuous and discrete homotopy operators are derived. It is shown that these formulas are equivalent to those in terms of Euler operators obtained from the variational complex.   The continuous homotopy operator automates integration by parts on the jet space. Its discrete analogue can be used in applications where summation by parts is essential. Several examples illustrate the use of the homotopy operators.   The calculus-based formulas for the homotopy operators are easy to implement in computer algebra systems such as Mathematica and Maple. The homotopy operators can be readily applied to the symbolic computation of conservation laws of nonlinear partial differential equations and differential-difference equations. 
W. Hereman, B. Deconinck, L.D. Poole