  The presented method of investigating the solutions to differential equations
is not new. Such an approach was developed by Cartan in his analysis of the
integrability of differential equations. Here this approach is outlined to
demonstrate the role of skew-symmetric differential forms.
  The role of skew-symmetric differential forms in a qualitative investigation
of the solutions to differential equations is conditioned by the fact that the
mathematical apparatus of these forms enables one to determine the conditions
of consistency for various elements of differential equations or for the system
of differential equations. This enables one, for example, to define the
consistency of the partial derivatives in the partial differential equations,
the consistency of the differential equations in the system of differential
equations, the conjugacy of the function derivatives and of the initial data
derivatives in ordinary differential equations and so on. The functional
properties of the solutions to differential equations are just depend on
whether or not the conjugacy conditions are satisfied.
