Plane functors

Tensor products of functors over the set category are studied in this work. The concept of a plane functor is defined and a criterion under which a functor is planar is given, by means of which the existence of injective envelopes in the functor category over the set category is proved.Translated from Matematicheskie Zametki, Vol. 19, No. 4, pp. 577–586, April, 1976.In conclusion, the author wishes to express his deep appreciation to A. Kh. Livshits for his deep attention to the work.