Lagmental Vicfred

Quasi-Coherent Sheaves on an Affine Scheme Recover Modules by Vicfred

Global sections and tilde give an equivalence between modules and quasi-coherent sheaves on an affine scheme. The formulas are more useful when each symbol has a job rather than merely decorating the theorem.

Definitions first

An \(A\)-module \(M\) produces a quasi-coherent sheaf \(\widetilde M\) on \(\operatorname{Spec}A\). Localisation gives its sections on basic opens and its stalks at primes.

$$ X=\operatorname{Spec}A $$

I keep the defining relation \(\mathsf D\) above the derived relation \(\mathsf C\). This exposes whether cancellation used \(x\ne0\) and whether the conclusion is canonical.

$$ A\text{-Mod}\simeq\operatorname{QCoh}(X) $$

A small case in full

The middle display is intentionally dense: it is where signs, bounds, multiplicities, or normalising factors are most likely to be lost.

$$ M\xrightarrow{\sim}\Gamma(X,\widetilde M),\qquad\widetilde{\Gamma(X,\mathcal F)}\xrightarrow{\sim}\mathcal F $$

The reusable statement

The abstraction earns its keep by explaining why the same computation reappears. The notation compresses repeated reasoning without erasing the hypothesis that licenses it.

$$ \begin{aligned} \mathsf{D}\;&:\quad X=\operatorname{Spec}A,\\[5pt] \mathsf{C}\;&:\quad A\text{-Mod}\simeq\operatorname{QCoh}(X). \end{aligned} $$

A nearby false statement

Sheafification repairs local compatibility, but it does not make every sheaf quasi-coherent. Affine module methods apply only to the quasi-coherent class.

$$ \boxed{\begin{gathered} \text{compact conclusion}\\[-2pt] A\text{-Mod}\simeq\operatorname{QCoh}(X) \end{gathered}} $$

The notation is dense, but it is doing honest work: every delimiter records scope and every index records dependence. Removing one should require a mathematical reason.

This article was posted on Thu 09 January 2014. Facts and circumstances may have changed since publication.
Please contact me before jumping to conclusions if something seems wrong or unclear.