Let $X$ be a ringed space. Recall that an $\mathcal{O}\_X$-module $M$ is called *coherent* if it is of finite presentation and for every open $U \subseteq X$ and any integer $n \ge 1$, the kernel of every morphism of $\mathcal{O}\_U$-modules $\mathcal O\_U^{\oplus n} \to M|\_U$ is of finite type. Coherent modules constitute an abelian category (in contrast to modules of finite presentation or just of finite type). See the Stacks project, [modules](http://www.math.columbia.edu/algebraic_geometry/stacks-git/modules.pdf), section 12. In general $\mathcal{O}\_X$ might be not coherent.



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> **Question.** If $M,N$ are coherent $O\_X$-modules, is it true that $M \otimes\_{\mathcal{O}\_X} N$ is coherent?
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I guess that this will be false in this generality. So let us restrict to schemes, w.l.o.g. affine schemes. Here a module $M$ over a ring $A$ is coherent if it is of finite presentation and every submodule of finite type is of finite presentation.



Tensor product of coherent modules
In the analytic category this is indeed true: if $\mathscr{F}$, $\mathscr{G}$ are coherent analytic sheaves on a complex space $X$, then $\mathscr{F} \otimes\_{\mathscr{O}\_X} \mathscr{G}$ is also coherent.


For a reference, look at [Grauert-Remmert, Coherent Analytic Sheaves], Proposition at the bottom of page 240.


It seems to me that their proof works also in the algebraic category, maybe you should check.


