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There is somewhat of symmetry in the definition of flat module and pure short exact sequence which can be made precise as follows. Let $\mathcal{R}$ be the class of all right $R$-modules, $\mathcal{S}$ be the class of all short exact sequences of left $R$-modules. For $\mathcal{A}\subseteq\mathca...
@CalvinKhor Yes I posted it on MO yesterday, but didn’t get any response 😢. Thanks for your concern, though. — Zhenhui Ding Jun 19 at 8:11
3 hours later…
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