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Let $A \subseteq B$ be commutative noetherian domains. Of course, if $M$ is an $A$-module, then $M \otimes_A B$ is a $B$-module. I am curious to know if there exist additional conditions on $A$ and $B$, such that every $B$-module $N$ is necessarily of the form $M \otimes_A B$ for some $A$-module $M$.

I do not mind to assume one or more of the following additional conditions: $A$ is a UFD (but I do not want to assume that $B$ is a UFD). $A$ is regular. $B$ is a complete intersection ring (but I do not want to assume that $B$ is regular). $B$ is a faithfully flat $A$-module. $B$ is a free $A$-module.

I once ran into a paper (unfortunately I cannot find it now) which calls such $N$ extendable (maybe that paper answers my curiosity?).

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    $\begingroup$ A module of the form $M\otimes B$ is sometimes called "relative free" (and a direct summand of such a module is called "relative projective"). You're asking for coniditions when every module is relative free. If $A$ is a field, then this means that every $B$-module is free which is more or less equivalent to being a principle ideal domain. $\endgroup$
    – Johannes Hahn
    Commented May 10, 2015 at 16:58
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    $\begingroup$ @JohannesHahn: what do you mean by "more or less equivalent"? If $B$ is a nonzero ring that is not a field and $I$ is a maximal ideal of $B$ then $B/I$ is a $B$-module that is not free. $\endgroup$
    – grghxy
    Commented May 10, 2015 at 17:04
  • $\begingroup$ Thank you both. Please can you recommend books/papers dealing with relative free modules? $\endgroup$
    – user237522
    Commented May 10, 2015 at 18:08
  • $\begingroup$ Hochschild, Relative homological algebra, Trans. Amer. Math. Soc. 1956. $\endgroup$
    – Vinteuil
    Commented May 11, 2015 at 8:55

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An example: if $B$ is the henselization of a local ring $A$, then for any finite type $B$-module $N$ there exists a finite type $A$-module $M$ such that $N$ is a direct summand of $M\otimes_AB$. You can find this result and some similar others in http://arxiv.org/pdf/0707.4197v3.pdf

I think there are more recent papers on this question maybe by some of the same authors, but I cannot remmenber more precisely.

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  • $\begingroup$ Great! I think the results in the above mentioned paper are very close to what I was looking for. $\endgroup$
    – user237522
    Commented May 11, 2015 at 13:56

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