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$$\begin{array}{ccccc} \text{schemes} & \longrightarrow & \text{locally ringed spaces} & \longrightarrow & \text{ringed spaces} \\ | && | && | \\ \text{quasi-coherent sheaves} & \longrightarrow & \text{?} & \longrightarrow & \text{module sheaves}\end{array}$$

What do you suggest for $?$, fitting into this picture?

This is a soft question, but perhaps I can make it more precise: I would like to know if there is any reasonable substack $\mathsf{LMod}$ of $\mathsf{Mod} : \mathsf{LRS} \to \mathsf{SymMonCat}^{\mathrm{op}}$ such that $\mathsf{LMod}(X)$ preserves "much" of the structure of $X$ (in particular we cannot just use the forgetful functor $\mathsf{LRS} \to \mathsf{RS}$). I would like to see something different from quasi-coherent or coherent sheaves, which really used the local rings. For example, when $x \in \overline{\{y\}}$, one can require that the canonical homomorphism $M_x \otimes_{\mathcal{O}_{X,x}} \mathcal{O}_{X,y} \to M_y$ is an isomorphism modulo $(\mathfrak{m}_y)^n$, but this is a little bit weak.

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Can't you also consider quasicoherent sheaves over locally ringed spaces? – Qiaochu Yuan Feb 27 '13 at 17:31
Hi Martin, Isn't there a notion of quasi-coherent sheaf on any ringed space? – Bruno Joyal Feb 27 '13 at 17:32
Being quasicoherent meand that on any open subset which is an affine scheme, the sheaf has a certain structure. This does not seem like a very useful property for an arbitrary locally ringed space (as there might be no such subspaces) – Tobias Kildetoft Feb 27 '13 at 17:42
@Qiaochu and Bruno: Of course you are right. But I want to know if there is any notion which is more "adapted" to the specific notion of locally ringed spaces. Probably this is not so well-known ... – Martin Brandenburg Feb 27 '13 at 17:43
@Tobias: See EGAI or the Stacks Project for the correct definition of quasi-coherent sheaves (which also applies to arbitrary ringed spaces, and is meaningful). – Martin Brandenburg Feb 27 '13 at 17:43

For every open $U \subset X$ and every $f \in \mathcal{O}_X(U)$ we can consider the open set $U_f$ of points $x$ of $U$ where $f$ does not map to the maximal ideal of $\mathcal{O}_{X, x} = \mathcal{O}_{U, x}$.

In the schemes case, if $U$ is quasi-compact and quasi-separated, then a quasi-coherent sheaf $\mathcal{F}$ has the property that $\mathcal{F}(U_f) = \mathcal{F}(U)_f$, see for example Lemma Tag 01P7. If $U$ is general, then I think we can still conclude that

  1. if $s \in \mathcal{F}(U)$ restricts to zero over $U_f$, then for every $x \in U$ there exists an open neighbourhood $x \in W \subset U$ and integer $n \geq 0$ such that $f^ns|_W = 0$

  2. if $t \in \mathcal{F}(U_f)$, then for every $x \in U$ there exists an open neighbourhood $x \in W \subset U$, a section $s \in \mathcal{F}(W)$, and an integer $n \geq 0$ such that $s|_{W_f} = f^nt|_{W_f}$.

Namely, we can just take $W$ to be an affine open neighbourhood of $x$ and apply the previous result. (There are variants of 1 and 2 using open coverings so that you can generalize to locally ringed topoi if you like.)

So a possibility would be to consider sheaves of $\mathcal{O}_X$-modules on a locally ringed space $(X, \mathcal{O}_X)$ which satisfy conditions 1 and 2.

On a scheme you'd recover the usual quasi-coherent modules (didn't check all details but it seems obvious -- please correct me if I am wrong). For a general locally ringed space I think you get a different notion than the usual quasi-coherent sheaves (for example for the real line endowed with continuous functions, it appears that the structure sheaf doesn't satisfy condition 2). In fact, I am not at all sure this is something interesting for any type of locally ringed space (or topos) different from a scheme or algebraic space or algebraic stack.

Actually, I think there are many natural properties one can impose on $\mathcal{O}_X$-modules which, when $X$ is a scheme, give the class of quasi-coherent modules. One is the condition above. Another one is that $X$ should have a covering $X = \bigcup U_i$ such that $\mathcal{F}|_{U_i}$ is associated to a $\mathcal{O}_X(U_i)$-module (as in Lemma Tag 01BH). Finally, there is the definition of quasi-coherent modules. But presumably there are many others.

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Interesting idea. But I want $\mathsf{LMod}(X)$ to be a symmetric monoidal category with unit and tensor product induced by $\mathsf{Mod}(X)$. So in particular $\mathcal{O}_X \in \mathsf{LMod}(X)$. But as you say, this is not clear for 1. and 2. – Martin Brandenburg Mar 29 '14 at 19:18

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