Transgression map

In algebraic topology, a transgression map is a way to transfer cohomology classes.
It occurs, for example in the inflation-restriction exact sequence in group cohomology, and in integration in fibers. It also naturally arises in many spectral sequences; see spectral sequence#Edge maps and transgressions.
Inflation-restriction exact sequence.
The transgression map appears in the inflation-restriction exact sequence, an exact sequence occurring in group cohomology. Let "G" be a group, "N" a normal subgroup, and "A" an abelian group which is equipped with an action of "G", i.e., a homomorphism from "G" to the automorphism group of "A". The quotient group formula_1 acts on 
Then the inflation-restriction exact sequence is:
The transgression map is the map formula_4.
Transgression is defined for general formula_5, 
only if formula_7 for formula_8.