Faithfully flat descent

Faithfully flat descent is a technique from algebraic geometry, allowing one to draw conclusions about objects on the target of a faithfully flat morphism. Such morphisms, that are flat and surjective, are common, one example coming from an open cover.

In practice, from an affine point of view, this technique allows one to prove some statement about a ring or scheme after faithfully flat base change.

"Vanilla" faithfully flat descent is generally false; instead, faithfully flat descent is valid under some finiteness conditions (e.g., quasi-compact or locally of finite presentation).

A faithfully flat descent is a special case of Beck's monadicity theorem.^[1]

Basic form[]

Let ${\displaystyle A\to B}$ be a faithfully flat ring homomorphism. Given an ${\displaystyle A}$-module ${\displaystyle M}$, we get the ${\displaystyle B}$-module ${\displaystyle N=M\otimes _{A}B}$ and because ${\displaystyle A\to B}$ is faithfully flat, we have the inclusion ${\displaystyle M\hookrightarrow M\otimes _{A}B}$. Moreover, we have the isomorphism ${\displaystyle \varphi :N\otimes B{\overset {\sim }{\to }}N\otimes B}$ of ${\displaystyle B^{\otimes 2}}$-modules that is induced by the isomorphism ${\displaystyle B^{\otimes 2}\simeq B^{\otimes 2},x\otimes y\mapsto y\otimes x}$ and that satisfies the cocycle condition:

${\displaystyle \varphi ^{1}=\varphi ^{0}\circ \varphi ^{2}}$

where ${\displaystyle \varphi ^{i}:N\otimes B^{\otimes 2}{\overset {\sim }{\to }}N\otimes B^{\otimes 2}}$ are given as:^[2]

${\displaystyle \varphi ^{0}(n\otimes b\otimes c)=\rho ^{1}(b)\varphi (n\otimes c)}$

${\displaystyle \varphi ^{1}(n\otimes b\otimes c)=\rho ^{2}(b)\varphi (n\otimes c)}$

${\displaystyle \varphi ^{2}(n\otimes b\otimes c)=\varphi (n\otimes b)\otimes c}$

with ${\displaystyle \rho ^{i}(x)(y_{0}\otimes \cdots \otimes y_{r})=y_{0}\cdots y_{i-1}\otimes x\otimes y_{i}\cdots y_{r}}$. Note the isomorphisms ${\displaystyle \varphi ^{i}:N\otimes B^{\otimes 2}{\overset {\sim }{\to }}N\otimes B^{\otimes 2}}$ are determined only by ${\displaystyle \varphi }$ and do not involve ${\displaystyle M.}$

Now, the most basic form of faithfully flat descent says that the above construction can be reversed; i.e., given a ${\displaystyle B}$-module ${\displaystyle N}$ and a ${\displaystyle B^{\otimes 2}}$-module isomorphism ${\displaystyle \varphi :N\otimes B{\overset {\sim }{\to }}N\otimes B}$ such that ${\displaystyle \varphi ^{1}=\varphi ^{0}\circ \varphi ^{2}}$, an invariant submodule:

${\displaystyle M=\{n\in N|\varphi (n\otimes 1)=n\otimes 1\}\subset N}$

is such that ${\displaystyle M\otimes B=N}$.^[3]

Zariski descent[]

The Zariski descent refers simply to the fact that a quasi-coherent sheaf can be obtained by gluing those on a (Zariski-)open cover. It is a special case of a faithfully flat descent but is frequently used to reduce the descent problem to the affine case.

In details, let ${\displaystyle {\mathcal {Q}}coh(X)}$ denote the category of quasi-coherent sheaves on a scheme X. Then Zariski descent states that, given quasi-coherent sheaves ${\displaystyle F_{i}}$ on open subsets ${\displaystyle U_{i}\subset X}$ with ${\displaystyle X=\bigcup U_{i}}$ and isomorphisms ${\displaystyle \varphi _{ij}:F_{i}|_{U_{i}\cap U_{j}}{\overset {\sim }{\to }}F_{j}|_{U_{i}\cap U_{j}}}$ such that (1) ${\displaystyle \varphi _{ii}=\operatorname {id} }$ and (2) ${\displaystyle \varphi _{ik}=\varphi _{jk}\circ \varphi _{ij}}$ on ${\displaystyle U_{i}\cap U_{j}\cap U_{k}}$, then exists a unique quasi-coherent sheaf ${\displaystyle F}$ on X such that ${\displaystyle F|_{U_{i}}\simeq F_{i}}$ in a compatible way (i.e., ${\displaystyle F|_{U_{j}}\simeq F_{j}}$ restricts to ${\displaystyle F|_{U_{i}\cap U_{j}}\simeq F_{i}|_{U_{i}\cap U_{j}}{\overset {\varphi _{ij}}{\underset {\sim }{\to }}}F_{j}|_{U_{i}\cap U_{j}}}$).^[4]

In a fancy language, the Zariski descent states that, with respect to the Zariski topology, ${\displaystyle {\mathcal {Q}}coh}$ is a stack; i.e., a category ${\displaystyle {\mathcal {C}}}$ equipped with the functor ${\displaystyle p:{\mathcal {C}}\to }$ the category of (relative) schemes that has an effective descent theory. Here, let ${\displaystyle {\mathcal {Q}}coh}$ denote the category consisting of pairs ${\displaystyle (U,F)}$ consisting of a (Zariski)-open subset U and a quasi-coherent sheaf on it and ${\displaystyle p}$ the forgetful functor ${\displaystyle (U,F)\mapsto U}$.

Descent for quasi-coherent sheaves[]

There is a succinct statement for the major result in this area: (the prestack of quasi-coherent sheaves over a scheme S means that, for any S-scheme X, each X-point of the prestack is a quasi-coherent sheaf on X.)

The proof uses and the faithfully flat descent in the affine case.

Here "quasi-compact" cannot be eliminated; see https://mathoverflow.net/questions/127362/counter-example-to-faithfully-flat-descent/

See also[]

• Amitsur complex
• Hilbert scheme
• Quot scheme

Notes[]

References[]

• SGA 1, Ch VIII – this is the main reference
• Hartshorne, Robin (1977), Algebraic Geometry, Graduate Texts in Mathematics, 52, New York: Springer-Verlag, ISBN 978-0-387-90244-9, MR 0463157
• Street, Ross (20 Mar 2003). "Categorical and combinatorial aspects of descent theory". arXiv:math/0303175. (a detailed discussion of a 2-category)
• Angelo Vistoli, Notes on Grothendieck topologies, fibered categories and descent theory (Updated September 2, 2008)
• Waterhouse, William (1979), Introduction to affine group schemes, Graduate Texts in Mathematics, 66, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-4612-6217-6, ISBN 978-0-387-90421-4, MR 0547117