Wednesday, 3 December 2008

ag.algebraic geometry - extending local modules to global modules

Let X be a scheme and xinX. Consider the functor



textQcoh(X)tomathcalOX,xtextMod,MmapstoMx.



Does it have a right-inverse? I.e. is there, for every mathcalOX,x-module N a (functorial) quasi-coherent mathcalOX-module M such that there is a natural isomorphism MxcongN?



If X is quasi-separated, the answer is yes. First observe that, if X is affine, the direct image with respect to textSpec(mathcalOX,x)toX works. Now if X is quasi-separated, use the affine case to extend N to a quasi-coherent module on an open affine neighborhood U of x, and then take the direct image with respect to UtoX. This works since UtoX is a quasi-compact, quasi-separated morphism.



In the general case, note that direct images don't work, but this does not disprove the existence of the functor I'm looking for. The question is motivated by this one, which is still unsolved.



If mathfrakmxsubseteqtextrad(textAnn(N)), then the direct image with respect to textSpec(mathcalOX,x/textAnn(N))toX (this is then an affine morphism!) works.



EDIT: A stronger, but more natural question is the following: Let UsubseteqX an open subset. Does then res:textQcoh(X)totextQcoh(U) have a right inverse? As I said, this is clear if UtoX is a quasi-compact morphism. In general, a transfinite recursion on the length of an affine cover shows that it is enough to consider the case that X is affine, but UsubseteqX arbitrary. Then everything is ok when U is quasi-compact. In general, you can write U as a directed union of quasi-compact subsets of X. Does this help somehow?



EDIT 2: There is a right adjoint textQcoh(U)totextQcoh(X) to the restriction functor due to abstract reasons. If X is affine, it maps N to the module associated to Gamma(U,N), where the latter is considered as a Gamma(X,mathcalOX)-module. However, this fails to be an extension of N, i.e. the counit widetildeGamma(U,N)|UtoN is no isomorphism in general (I have an explicit counterexample). Thus, the desired right-inverse (if it exists) won't be a right adjoint.



EDIT 3: Here is a class of examples where extension works. Assume X is affine and UsubseteqX open can be written as coprodiinIUi with Ui affine. If MintextQcoh(U) and Mi:=Gamma(Ui,M), then widetildeGamma(U,M)=widetildeprodiinIMiintextQcoh(X) is not an extension of M, but widetildebigoplusiinIMi works.



EDIT 4: Let U,X,M as in edit 3. Then an application of Zorn's lemma shows that M can be extended to a maximal open subset UsubseteqVsubseteqX. Then for every open affine WsubseteqX, either VcapW=W or VcapW is not quasi-compact. In particular, V is ("very") dense. For example, mathbbAinfty0subseteqmathbbAinfty is such a dense subset. But I don't know if here extension works. I've already looked at several examples, but have not found out anything ..



EDIT 5: If dim(X)=0, then extension works.



Please let me know if you have any ideas!

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