Monday, 18 November 2013

ac.commutative algebra - Does every projective A/I-module come from A?

Let $A$ be a Noetherian commutative ring and $I$ an ideal in $A$. It is pretty much trivial to see that every free $A/I$-Module is obtained from a free $A$-module by tensoring over $A$ with $A/I$: Just choose preimages for every generator and take the free module generated by them. I'm wondering whether the same remains true if I replace free by projective and finitely generated.



One way to prove this would be the following: Let $X=Spec A$, $Z=Spec(A/I)$, and $U=Z^c$. If there were something like a an exact sequence



$K_0(U) rightarrow K_0(X) rightarrow K_0(Z) rightarrow 0$



the claim would follow, right? Does such a sequence exist? If I'm not mistaken, such a sequence exists if A is a Dedekind ring. Just to make it clear, here is the precise question:



Question: Given a Noetherian commutative ring $A$, an ideal $I$ and a projective, finitely generated $A/I$-module $M$, does there exist a projective, finitely generated $A$-module $N$ such that $N/IN=M$?

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