Quasi-coherent module given by modules and compatibility conditions in the language of commutative algebra - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T02:21:40Z http://mathoverflow.net/feeds/question/40491 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/40491/quasi-coherent-module-given-by-modules-and-compatibility-conditions-in-the-langua Quasi-coherent module given by modules and compatibility conditions in the language of commutative algebra Martin Brandenburg 2010-09-29T16:16:00Z 2010-09-29T18:11:36Z <h2>Short question</h2> <p>Can we describe a quasi-coherent module on a scheme by usual modules with respect to an affine cover, which satisfy some compatibility conditions, which can be formulated in the language of commutative algebra (actually tensor products of modules)? Thus, restrictions to non-affine subsets are not allwed. </p> <h2>Example</h2> <p>If the scheme is separated, the answer is of course yes: Choose an affine cover $U_i$ of our scheme $X$, thus also $U_i \cap U_j$ is affine. If $M_i$ is a $\mathcal{O}(U_i)$-module such that we have isomorphisms $\phi_{ij} : M_i \otimes_{\mathcal{O}(U_i)} \mathcal{O}(U_i \cap U_j) \cong M_j \otimes_{\mathcal{O}(U_j)} \mathcal{O}(U_i \cap U_j)$ of $\mathcal{O}(U_i \cap U_j)$-modules, which satisfy the cocycle conditions, namely $\phi_{ii} = id$ and $\phi_{ijk} = \phi_{kji} \phi_{ikj}$ where $\phi_{ijk} = \phi_{ij} \otimes_{\mathcal{O}(U_i \cap U_j)} \mathcal{O}(U_i \cap U_j \cap U_k)$. Then the quasi-coherent modules $\widetilde{M_i}$ on $U_i$ can be glued to a quasi-coherent module on $X$.</p> <h2>Long question</h2> <p>In the general case, there is an affine cover $U_i \cap U_j = \cup_k W_k$, such that $W_k$ is basic-open in $U_i$ and $U_j$ simulteneously. In this setting we should use isomorphisms $\phi_{ijk} : M_i \otimes_{\mathcal{O}(U_i)} \mathcal{O}(W_k) \cong M_j \otimes_{\mathcal{O}(U_j)} \mathcal{O}(W_k)$. However, in order to formulate the cocycle condition in affine terms, we actually have to cover triple overlaps $U_i \cap U_j \cap U_k$ by affines $R_n$, such that $R_n$ is basic-open in $U_i,U_j,U_j$ and then cover the $U_i \cap U_j$ with these $R_n$, where we vary $k$. Thus we assume isomorphisms $\phi_{ijkn} : M_i \otimes_{\mathcal{O}(U_i)} \mathcal{O}(R_n) \cong M_j \otimes_{\mathcal{O}(U_j)} \mathcal{O}(R_n)$ and may formulate the cocycle condition. Now in order to glue these isomorphisms to an isomorphism on $U_i \cap U_j$ between $\widetilde{M_i}$ and $\widetilde{M_j}$, we also have to ensure that there is some intersection compatibility when we vary $k$. However, this cannot be formulated with the given data.</p> <p>Before I end up defining refinements of these affine covers by transfinite induction ;-), I thought it would be better to ask here if there is an easy way which I overlook.</p> http://mathoverflow.net/questions/40491/quasi-coherent-module-given-by-modules-and-compatibility-conditions-in-the-langua/40501#40501 Answer by t3suji for Quasi-coherent module given by modules and compatibility conditions in the language of commutative algebra t3suji 2010-09-29T17:31:58Z 2010-09-29T17:31:58Z <p>I think the explicit description that you suggest can be wrapped up as follows.</p> <p>For every $i,j$, let $C_{ij}$ be a family of indexes such that $$U_i\cap U_j=\bigcup_{a\in C_{ij}} W_a,$$ with $W_a$ being basic open in $U_i$ and in $U_j$. We may assume that $C_{ii}$ is a singleton set and $C_{ij}=C_{ji}$. Let $\phi_{ij,a}$ be as in your question, $a\in C_{ij}$. Impose the cocycle condition $$\phi_{ij,a}\circ\phi_{jk,b}=\phi_{ik,c}$$ on $W_a\cap W_b\cap W_c$ for all $a\in C_{ij}$, $b\in C_{jk}$, $c\in C_{ik}$. Notice that $W_a\cap W_b\cap W_c$ is a basic open in $W_a$, $W_b$, $W_c$, and also in $U_i$, $U_j$, $U_k$, so this is an identity in algebraic terms, as required.</p> <p>A more natural description:</p> <p>A quasicoherent sheaf on $X$ is given by a $\Gamma(U,O)$-module $M_U$ for every affine open $U\subset X$ and an isomorphism $\phi_{U,V}:M_U|_V\to M_V$ whenever $V\subset U$ is a basic open, such that $\phi_{U,V}$ satisfies obvious cocycle condition on triples $W\subset V\subset U$.</p>