3
$\begingroup$

This question arises from my discussion with a Master student. It concerns with the following situation: let $\phi: R \to S$ be a homomorphism between Noetherian commutative rings. Suppose the $R$-module structure of $S$ has a presentation matrix with all entries in the Jacobson radical of $R$ (so $S$, as $R$-module, is the cokernel of such matrix). Let $M$ be a finitely generated, projective $R$-module.

Question: If $M \otimes_R S$ is $S$-free, must $M$ be $R$-free?

Remark: this is trivially true if $R$ is semi-local. It is easy and well-known if $\phi$ is surjective. Without some conditions on $S$, the assertion is false, for example if we take $S$ to be a residue field of a maximal ideal in $R$ and $M$ be some projective, non-free module.

This kind of statement sounds like it should be in Bourbaki or EGA or the stack project (if it is true!). Does anyone know a proof or counter example?

Improve this question
$\endgroup$
9
  • $\begingroup$ Your assumption on $S$ is satisfied in particular if $S$ is a free $R$-module. I wouldn't be surprised if you could find a counterexample in this case (e.g. with $R$ a ring of algebraic integers, and $M$ invertible?) $\endgroup$
    – Laurent Moret-Bailly
    Commented Mar 9, 2011 at 13:47
  • $\begingroup$ Dear Laurent, that sounds promising! So we want $S$ to be $R$-free, $Pic(S)$ is trivial but $Pic(R)$ is not? $\endgroup$
    – Hailong Dao
    Commented Mar 9, 2011 at 14:02
  • 3
    $\begingroup$ Actually, $R=\mathbb R[x,y]/(x^2+y^2-1)$, $S=R\otimes \mathbb C$ seems to work. $\endgroup$
    – Hailong Dao
    Commented Mar 9, 2011 at 14:19
  • 1
    $\begingroup$ Nice and simple! $\endgroup$
    – Laurent Moret-Bailly
    Commented Mar 9, 2011 at 14:41
  • $\begingroup$ So, I guess $Pic(R)$ doesn't need to be non-trivial, then? (Since all line bundles over $S^2$ are free...) $\endgroup$
    – Andrew Parker
    Commented Mar 9, 2011 at 17:08

1 Answer 1

Reset to default
1
$\begingroup$

Let me answer and accept this in CW so that it will not be bumped periodically as not answered by the software. It was hoped that the case of $\phi$ surjective can be generalized, but as Laurent pointed out in the comments, one can not hope to get any reasonable statement.

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .