 A Herbrand module is an "A" for which the cohomology groups are finite.In this case, the Herbrand quotient "h"("G","A") is defined to be the quotient
of the order of the even and odd cohomology groups.The quotient may be defined for a pair of endomorphisms of an Abelian group, "f" and "g", which satisfy the condition "fg" = "gf" = 0.