Identical and Nonidentical Relations. Nondegenerate and Degenerate   Transformations. (Properties of skew-symmetric differential forms)
  Identical relations occur in various branches of mathematics and mathematical physics. The Cauchy-Riemann relations, characteristical and canonical relations, the Bianchi identities and others are examples of identical relations. It can be shown that all these relations express either the conditions of closure of exterior (skew-symmetric) differential forms and corresponding dual forms or the properties of closed exterior forms. Since the closed differential forms are invariant under all transformations, which conserve the differential (these are gauge transformations: unitary, canonical, gradient and others), from this it follows that identical relations are a mathematical representation of relevant invariant and covariant objects.   The theory of exterior differential forms cannot answer the question of how do invariant objects appear and what does these objects generate?   The answer to this question can be obtained using the skew-symmetric differential forms, which possess the evolutionary properties. The mathematical apparatus of such evolutionary forms contains nonidentical relations, from which the identical relations corresponding to invariant objects are obtained with the help of degenerate transformations. Due to such potentialities, the mathematical apparatus of skew-symmetrical differential forms enables one to describe discrete transitions, evolutionary processes and generation of various structures. 
L. I. Petrova