Past exam of the mathematics course of the University of Cambridge / 2010 / iii / Paper 3 / 5 / 3 / Solution

For an ideal $I$, the canonical multiplication maphas image $IM$. Consequently the asserted isomorphism is the natural surjection $I{\otimes }_{A}M\twoheadrightarrow IM$ being injective. This is the flatness criterion using ideals, whose proof follows from the extension criterion above.

Under the character Tensor-hom adjunction, the dual map ${\mu }_I^{\ast }$ identifies withIndeed, for $\varphi \in M^{\ast }$, its restriction corresponds to $i\otimes m\mapsto \varphi (im)$. If $M^{\ast }$ is injective, this restriction is surjective, so the cogenerator criterion makes ${\mu }_I$ injective. Conversely, if every ${\mu }_I$ is injective, every restriction map in this display is surjective. The proved Baer criterion then makes $M^{\ast }$ injective. ThusIn particular, (1) also implies (3) directly by tensoring $I\hookrightarrow A$. No finite-generation condition on $I$, and no Noetherian condition on $A$, has been inserted.