Suppose "R" is a ring, and denoted by "R"-Mod the category of left "R"-modules and by Mod-"R" the category of right "R"-modules (if "R" is commutative, the two categories coincide). Fix a module "B" in "R"-Mod. For "A" in Mod-"R", set "T"("A") = "A"⊗"R""B". Then "T" is a right exact functor from Mod-"R" to the category of abelian groups Ab (in the case when "R" is commutative, it is a right exact functor from Mod-"R" to Mod-"R") and its left derived functors "LnT" are defined. We set