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so I am learning about flat modules and I found this criterion for flatness. An $R$-module $M$ is flat iff for every finitely generated ideal $I$, we get that the multiplication map $I\otimes_R M\rightarrow M$ is an injection (Eisenbud Proposition 6.1)

I am confused because say we have any $M$ (an $R$ module), and say that $i\otimes m\mapsto 0$. This means that $im=0$, but wouldn't this imply that $i\otimes m=1\otimes im=1\otimes 0=0$, so the kernel is trivial, and hence the map is injective always?

I am obviously doing something wrong, so I'd appreciate anyone that could clarify things a bit!

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    $\begingroup$ Also: Not every element of the tensor product is a pure tensor. The elements are sums of these. $\endgroup$ – Martin Brandenburg Oct 13 '13 at 13:24
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You can't write $i\otimes m=1\otimes im$ unless $1\in I$, i.e., unless $I=R$.

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  • $\begingroup$ Thanks. That was very dumb of me. $\endgroup$ – Daniel Montealegre Oct 13 '13 at 0:59

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