Apparently this principle was first formulated for left modules over the group algebra $A=kG$ of a finite group, where $k$ is a field of characteristic $p>0$ dividing $|G|$. (See Exercise 2 on p. 426 of Curtis & Reiner, *Representation Theory of Finite Groups and Associative Algebras*, 1962.) Here the Hopf algebra structure of *A* yields a natural left module structure on the tensor product of two left modules over *k*.

By the mid-1970s similar tensor product behavior was observed in other special cases for left *A*-modules and their tensor products, where *A* is a Hopf algebra over a commutative ring *k*: (1) the (finite dimensional) restricted enveloping algebra of a restricted Lie algebra $\mathfrak{g}$ over a field of prime characteristic; (2) more generally the hyperalgebra of a higher Frobenius kernel when $\mathfrak{g}$ is the Lie algebra of a reductive algebraic group; (3) the universal enveloping algebra of a Kac-Moody algebra in characteristic 0; (4) the full hyperalgebra of a reductive algebraic group in prime characteristic (with "projective" replaced by "injective" as in J.C. Jantzen's book *Representations of Algebraic Groups*, I.3). Relevant references:

B. Pareigis, *Kohomologie von p-Lie Algebren*, Math. Z. 104 (1968); Lemma 2.5

J.E. Humphreys, *Projective modules for SL(2,q)*, J. Algebra 25 (1973); Thms. 1, 2 (and note
added in proof referring to Pareigis)

J.E. Humphreys, *Ordinary and modular representations of Chevalley groups*, Springer
Lect. Notes in Math. 528 (1976); Appendix T (following Sweedler's suggestion)

H. Garland and J. Lepowsky, *Lie algebra homology and the Macdonald-Kac formulas*, Invent. Math. 34 (1976); 1.7 and Remark

J.E. Humphreys, *On the hyperalgebra of a semisimple algebraic group*, in *Contributions to Algebra*, Academic Press, 1977; 3.1

The arguments here typically involve special cases of a general theorem suggested by Sweedler (and closely related to the "tensor identity" discussed in a recent MO post 37709 ): Let $A$ be a Hopf algebra (with antipode) over a commutative ring $k$, with Hopf subalgebra $B$ (possibly *k*). Given an $A$-module $M$ and a $B$-module $N$, there is a natural $A$-module isomorphism:
$$(A \otimes_B N) \otimes_k M \cong A \otimes_B (N \otimes_k M)$$ On the left side, *A* acts via comultiplication, while on the right it acts on the first factor.

Is this the optimal generality, and if so is there a textbook reference?