The Stacks project

Proof. Write $M = \mathop{\mathrm{colim}}\nolimits _{i \in I} M_ i$ and $N = \mathop{\mathrm{colim}}\nolimits _{j \in J} N_ j$ as directed colimits of finitely presented $R$-modules. Denote $f_{ii'} : M_ i \to M_{i'}$ and $g_{jj'} : N_ j \to N_{j'}$ the transition maps. Then $M_ i \otimes _ R N_ j$ is a finitely presented $R$-module (see Lemma 10.12.14), and $M \otimes _ R N = \mathop{\mathrm{colim}}\nolimits _{(i, j) \in I \times J} M_ i \otimes _ R M_ j$. Pick $(i, j) \in I \times J$. By the definition of a Mittag-Leffler module we have Proposition 10.88.6 (3) for both systems. In other words there exist $i' \geq i$ and $j' \geq j$ such that for every choice of $i'' \geq i$ and $j'' \geq j$ there exist maps $a : M_{i''} \to M_{i'}$ and $b : M_{j''} \to M_{j'}$ such that $f_{ii'} = a \circ f_{ii''}$ and $g_{jj'} = b \circ g_{jj''}$. Then it is clear that $a \otimes b : M_{i''} \otimes _ R N_{j''} \to M_{i'} \otimes _ R N_{j'}$ serves the same purpose for the system $(M_ i \otimes _ R N_ j, f_{ii'} \otimes g_{jj'})$. Thus by the characterization Proposition 10.88.6 (3) we conclude that $M \otimes _ R N$ is Mittag-Leffler. $\square$