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DESCENT0238Contents1. Introduction2. Descent data for quasi-coherent sheaves3. Descent for modules4. Descent for universally injective morphisms4.1. Category-theoretic preliminaries4.3. Universally injective morphisms4.14. Descent for modules and their morphisms4.23. Descent for properties of modules5. Fpqc descent of quasi-coherent sheaves6. Galois descent for quasi-coherent sheaves7. Descent of finiteness properties of modules8. Quasi-coherent sheaves and topologies9. Parasitic modules10. Fpqc coverings are universal effective epimorphisms11. Descent of finiteness and smoothness properties of morphisms12. Local properties of schemes13. Properties of schemes local in the fppf topology14. Properties of schemes local in the syntomic topology15. Properties of schemes local in the smooth topology16. Variants on descending properties17. Germs of schemes18. Local properties of germs19. Properties of morphisms local on the target20. Properties of morphisms local in the fpqc topology on the target21. Properties of morphisms local in the fppf topology on the target22. Application of fpqc descent of properties of morphisms23. Properties of morphisms local on the source24. Properties of morphisms local in the fpqc topology on the source25. Properties of morphisms local in the fppf topology on the source26. Properties of morphisms local in the syntomic topology on the source27. Properties of morphisms local in the smooth topology on the source28. Properties of morphisms local in the étale topology on the source29. Properties of morphisms étale local on source-and-target30. Properties of morphisms of germs local on source-and-target31. Descent data for schemes over schemes32. Fully faithfulness of the pullback functors33. Descending types of morphisms34. Descending affine morphismsThis is a chapter of the Stacks Project, version d9093438, compiled on Feb 26, 5454575858595960606971758082

DESCENT235. Descending quasi-affine morphisms36. Descent data in terms of sheaves37. Other chaptersReferences828384861. Introduction0239In the chapter on topologies on schemes (see Topologies, Section 1) we introducedZariski, étale, fppf, smooth, syntomic and fpqc coverings of schemes. In this chapterwe discuss what kind of structures over schemes can be descended through suchcoverings. See for example [Gro95a], [Gro95b], [Gro95e], [Gro95f], [Gro95c], and[Gro95d]. This is also meant to introduce the notions of descent, descent data,effective descent data, in the less formal setting of descent questions for quasicoherent sheaves, schemes, etc. The formal notion, that of a stack over a site, isdiscussed in the chapter on stacks (see Stacks, Section 1).2. Descent data for quasi-coherent sheaves023AIn this chapter we will use the convention where the projection maps pri : X . . . X X are labeled starting with i 0. Hence we have pr0 , pr1 : X X X,pr0 , pr1 , pr2 : X X X X, etc.023BDefinition 2.1. Let S be a scheme. Let {fi : Si S}i I be a family of morphismswith target S.(1) A descent datum (Fi , ϕij ) for quasi-coherent sheaves with respect to thegiven family is given by a quasi-coherent sheaf Fi on Si for each i I, anisomorphism of quasi-coherent OSi S Sj -modules ϕij : pr 0 Fi pr 1 Fj foreach pair (i, j) I 2 such that for every triple of indices (i, j, k) I 3 thediagram/ pr 2 Fkpr 0 Fi:pr 02 ϕikpr 01 ϕij pr 1 Fjpr 12 ϕjkof OSi S Sj S Sk -modules commutes. This is called the cocycle condition.(2) A morphism ψ : (Fi , ϕij ) (Fi0 , ϕ0ij ) of descent data is given by a familyψ (ψi )i I of morphisms of OSi -modules ψi : Fi Fi0 such that all thediagramspr 0 Fi ϕij / pr 1 Fjpr 0 ψi pr 0 Fi0pr 1 ψjϕ0ij / pr 1 F 0jcommute.SA good example to keep in mind is the following. Suppose that S Si is anopen covering. In that case we have seen descent data for sheaves of sets in Sheaves,Section 33 where we called them “glueing data for sheaves of sets with respect to thegiven covering”. Moreover, we proved that the category of glueing data is equivalent

DESCENT3to the category of sheaves on S. We will show the analogue in the setting abovewhen {Si S}i I is an fpqc covering.In the extreme case where the covering {S S} is given by idS a descent datumis necessarily of the form (F, idF ). The cocycle condition guarantees that theidentity on F is the only permitted map in this case. The following lemma showsin particular that to every quasi-coherent sheaf of OS -modules there is associateda unique descent datum with respect to any given family.023CLemma 2.2. Let U {Ui U }i I and V {Vj V }j J be families ofmorphisms of schemes with fixed target. Let (g, α : I J, (gi )) : U V bea morphism of families of maps with fixed target, see Sites, Definition 8.1. Let(Fj , ϕjj 0 ) be a descent datum for quasi-coherent sheaves with respect to the family{Vj V }j J . Then(1) The systemgi Fα(i) , (gi gi0 ) ϕα(i)α(i0 ) is a descent datum with respect to the family {Ui U }i I .(2) This construction is functorial in the descent datum (Fj , ϕjj 0 ).(3) Given a second morphism (g 0 , α0 : I J, (gi0 )) of families of maps with fixedtarget with g g 0 there exists a functorial isomorphism of descent data(gi Fα(i) , (gi gi0 ) ϕα(i)α(i0 ) ) ((gi0 ) Fα0 (i) , (gi0 gi00 ) ϕα0 (i)α0 (i0 ) ).Proof. Omitted. Hint: The maps gi Fα(i) (gi0 ) Fα0 (i) which give the isomorphism of descent data in part (3) are the pullbacks of the maps ϕα(i)α0 (i) by themorphisms (gi , gi0 ) : Ui Vα(i) V Vα0 (i) . Any family U {Si S}i I is a refinement of the trivial covering {S S} in aunique way. For a quasi-coherent sheaf F on S we denote simply (F Si , can) thedescent datum with respect to U obtained by the procedure above.023D023EDefinition 2.3. Let S be a scheme. Let {Si S}i I be a family of morphismswith target S.(1) Let F be a quasi-coherent OS -module. We call the unique descent on Fdatum with respect to the covering {S S} the trivial descent datum.(2) The pullback of the trivial descent datum to {Si S} is called the canonical descent datum. Notation: (F Si , can).(3) A descent datum (Fi , ϕij ) for quasi-coherent sheaves with respect to thegiven covering is said to be effective if there exists a quasi-coherent sheafF on S such that (Fi , ϕij ) is isomorphic to (F Si , can).SLemma 2.4. Let S be a scheme. Let S Ui be an open covering. Any descentdatum on quasi-coherent sheaves for the family U {Ui S} is effective. Moreover, the functor from the category of quasi-coherent OS -modules to the category ofdescent data with respect to U is fully faithful.Proof. This follows immediately from Sheaves, Section 33 and the fact that beingquasi-coherent is a local property, see Modules, Definition 10.1. To prove more we first need to study the case of modules over rings.

DESCENT43. Descent for modules023FLet R A be a ring map. By Simplicial, Example 5.5 this gives rise to a cosimplicial R-algebra//o/ A R A R AAoA AR/o/Let us denote this (A/R) so that (A/R)n is the (n 1)-fold tensor product of Aover R. Given a map ϕ : [n] [m] the R-algebra map (A/R) (ϕ) is the mapYYYa0 . . . an 7 ai ai . . . aiϕ(i) 0ϕ(i) 1ϕ(i) mwhere we use the convention that the empty product is 1. Thus the first few maps,notation as in Simplicial, Section 5, areδ01δ11σ00δ02δ12δ22σ01σ11:a0:a0:a0 a1:a0 a1:a0 a1:a0 a1: a0 a1 a2: a0 a1 a27 1 a07 a0 17 a0 a17 1 a0 a17 a0 1 a17 a0 a1 17 a0 a1 a27 a0 a1 a2and so on.An R-module M gives rise to a cosimplicial (A/R) -module (A/R) R M . In otherwords Mn (A/R)n R M and using the R-algebra maps (A/R)n (A/R)m todefine the corresponding maps on M R (A/R) .The analogue to a descent datum for quasi-coherent sheaves in the setting of modules is the following.023GDefinition 3.1. Let R A be a ring map.(1) A descent datum (N, ϕ) for modules with respect to R A is given by anA-module N and an isomorphism of A R A-modulesϕ : N R A A R Nsuch that the cocycle condition holds: the diagram of A R A R A-modulemapsN R A R A/ A R A R N6ϕ02ϕ01ϕ12(A R N R Acommutes (see below for notation).(2) A morphism (N, ϕ) (N 0 , ϕ0 ) of descent data is a morphism of A-modulesψ : N N 0 such that the diagramN R Aϕ/ A R Nϕ0 ψ idA N 0 R Ais commutative.idA ψ/ A R N 0

DESCENT5In the definition Pwe use the notation that Pϕ01 ϕ idA , ϕ12 idA ϕ, andϕ02 (n 1 1) ai 1 ni if ϕ(n 1) ai ni . All three are A R A R Amodule homomorphisms. Equivalently we haveϕij ϕ (A/R)1 ,2 )(A/R) (τij(A/R)222: [1] [2] is the map 0 7 i, 1 7 j. Namely, (A/R) (τ02)(a0 a1 ) where τij1a0 1 a1 , and similarly for the others .We need some more notation to be able to state the next lemma. Let (N, ϕ) be adescent datum with respect to a ring map R A. For n 0 and i [n] we setNn,i A R . . . R A R N R A R . . . R Awith the factor N in the ith spot. It is an (A/R)n -module. If we introduce themaps τin : [0] [n], 0 7 i then we see thatNn,i N (A/R)0 ,(A/R) (τin )(A/R)nnFor 0 i j n we let τij: [1] [n] be the map such that 0 maps to i and 1 toj. Similarly to the above the homomorphism ϕ induces isomorphismsϕnij ϕ (A/R)1 ,n)(A/R) (τij(A/R)n : Nn,i Nn,jof (A/R)n -modules when i j. If i j we set ϕnij id. Since these are allisomorphisms they allow us to move the factor N to any spot we like. And thecocycle condition exactly means that it does not matter how we do this (e.g., as acomposition of two of these or at once). Finally, for any β : [n] [m] we definethe morphismNβ,i : Nn,i Nm,β(i)as the unique (A/R) (β)-semi linear map such thatNβ,i (1 . . . n . . . 1) 1 . . . n . . . 1for all n N . This hints at the following lemma.023HLemma 3.2. Let R A be a ring map. Given a descent datum (N, ϕ) we canassociate to it a cosimplicial (A/R) -module N 2 by the rules Nn Nn,n and givenβ : [n] [m] setting we defineN (β) (ϕmβ(n)m ) Nβ,n : Nn,n Nm,m .This procedure is functorial in the descent datum.Proof. Here are the first few maps where ϕ(n 1) δ01δ11σ00δ02δ12δ22σ01σ11:N:N:A N:A N:A N:A N: A A N: A A N A N A N N A A N A A N A A N A N A NPnna0 na0 na0 na0 na 0 a1 na 0 a1 nαi xi7 P1 n7 α i xi7 a0 n7 1 a0 n7 Pa0 1 n7 a0 αi xi7 a0 a1 n7 a0 a1 n1Note that τ 2 δ 2 , if {i, j, k} [2] {0, 1, 2}, see Simplicial, Definition 2.1.ijk2We should really write (N, ϕ) .

DESCENT6with notation as in Simplicial, Section 5. We first verify the two properties σ00 δ01 id and σ00 δ11 id. The first one, σ00 δ01 id, is clear from the explicitdescription of the morphisms above. To prove the second relation we have touse the cocycle condition (because it does not holds for an arbitrary isomorphismϕ : N R A A R N ). Write p σ00 δ11 : N N . By the description of themaps above we deduce that p is also equal top ϕ id : N (N R A) (A R A) A (A R N ) (A R A) A NSinceϕ is an isomorphism we see that p is an isomorphism.PP Write ϕ(n 1) αi xi forcertainα Aandx N.Thenp(n) αi xi . Next, writeiiPϕ(xi 1) αij yj for certain αij A and yj N . Then the cocycle conditionsays thatXXαi αij yj αi 1 xi .PPPThis means that p(n) αi xi αi αij yj αi p(xi ) p(p(n)). Thus p is aprojector, and since it is an isomorphism it is the identity.To prove fully that N is a cosimplicial module we have to check all 5 types ofrelations of Simplicial, Remark 5.3. The relations on composing σ’s are obvious.The relations on composing δ’s come down to the cocycle condition for ϕ. In exactlythe same way as above one checks the relations σj δj σj δj 1 id. Finally,the other relations on compositions of δ’s and σ’s hold for any ϕ whatsoever. Note that to an R-module M we can associate a canonical descent datum, namely(M R A, can) where can : (M R A) R A A R (M R A) is the obvious map:(m a) a0 7 a (m a0 ).023ILemma 3.3. Let R A be a ring map. Let M be an R-module. The cosimplicial (A/R) -module associated to the canonical descent datum is isomorphic to thecosimplicial module (A/R) R M . Proof. Omitted.023JDefinition 3.4. Let R A be a ring map. We say a descent datum (N, ϕ) iseffective if there exists an R-module M and an isomorphism of descent data from(M R A, can) to (N, ϕ).Let R A be a ring map. Let (N, ϕ) be a descent datum. We may take thecochain complex s(N ) associated with N (see Simplicial, Section 25). It has thefollowing shape:N A R N A R A R N . . .We can describe the maps. The first map is the mapn 7 1 n ϕ(n 1).The second map on pure tensors has the valuesa n 7 1 a n a 1 n a ϕ(n 1).It is clear how the pattern continues.In the special case where N A R M we see that for any m M the element1 m is in the kernel of the first map of the cochain complex associated to thecosimplicial module (A/R) R M . Hence we get an extended cochain complex023K(3.4.1)0 M A R M A R A R M . . .

DESCENT7Here we think of the 0 as being in degree 2, the module M in degree 1, themodule A R M in degree 0, etc. Note that this complex has the shape0 R A A R A A R A R A . . .when M R.023LLemma 3.5. Suppose that R A has a section. Then for any R-module M theextended cochain complex (3.4.1) is exact.Proof. By Simplicial, Lemma 28.5 the map R (A/R) is a homotopy equivalenceof cosimplicial R-algebras (here R denotes the constant cosimplicial R-algebra).Hence M (A/R) R M is a homotopy equivalence in the category of cosimplicialR-modules, because R M is a functor from the category of R-algebras to thecategory of R-modules, see Simplicial, Lemma 28.4. This implies that the inducedmap of associated complexes is a homotopy equivalence, see Simplicial, Lemma28.6. Since the complex associated to the constant cosimplicial R-module M is thecomplex1 /0 /1 /0 /MMMM .Mwe win (since the extended version simply puts an extra M at the beginning).023M Lemma 3.6. Suppose that R A is faithfully flat, see Algebra, Definition 39.1.Then for any R-module M the extended cochain complex (3.4.1) is exact.Proof. Suppose we can show there exists a faithfully flat ring map R R0 suchthat the result holds for the ring map R0 A0 R0 R A. Then the result followsfor R A. Namely, for any R-module M the cosimplicial module (M R R0 ) R0(A0 /R0 ) is just the cosimplicial module R0 R (M R (A/R) ). Hence the vanishingof cohomology of the complex associated to (M R R0 ) R0 (A0 /R0 ) implies thevanishing of the cohomology of the complex associated to M R (A/R) by faithfulflatness of R R0 . Similarly for the vanishing of cohomology groups in degrees 1 and 0 of the extended complex (proof omitted).But we have such a faithful flat extension. Namely R0 A works because the ringmap R0 A A0 A R A has a section a a0 7 aa0 and Lemma 3.5 applies. Here is how the complex relates to the question of effectivity.039WLemma 3.7. Let R A be a faithfully flat ring map. Let (N, ϕ) be a descentdatum. Then (N, ϕ) is effective if and only if the canonical mapA R H 0 (s(N )) Nis an isomorphism.Proof. If (N, ϕ) is effective, then we may write N A R M with ϕ can. Itfollows that H 0 (s(N )) M by Lemmas 3.3 and 3.6. Conversely, suppose the mapof the lemma is an isomorphism. In this case set M H 0 (s(N )). This is anR-submodule of N , namely M {n N 1 n ϕ(n 1)}. The only thing tocheck is that via the isomorphism A R M N the canonical descent data agreeswith ϕ. We omit the verification. 039XLemma 3.8. Let R A be a faithfully flat ring map, and let R R0 be faithfullyflat. Set A0 R0 R A. If all descent data for R0 A0 are effective, then so areall descent data for R A.

DESCENT8Proof. Let (N, ϕ) be a descent datum for R A. Set N 0 R0 R N A0 A N ,and denote ϕ0 idR0 ϕ the base change of the descent datum ϕ. Then (N 0 , ϕ0 )is a descent datum for R0 A0 and H 0 (s(N 0 )) R0 R H 0 (s(N )). Moreover, themap A0 R0 H 0 (s(N 0 )) N 0 is identified with the base change of the A-modulemap A R H 0 (s(N )) N via the faithfully flat map A A0 . Hence we concludeby Lemma 3.7. Here is the main result of this section. Its proof may seem a little clumsy; for amore highbrow approach see Remark 3.11 below.023NProposition 3.9. Let R A be a faithfully flat ring map. Then(1) any descent datum on modules with respect to R A is effective,(2) the functor M 7 (A R M, can) from R-modules to the category of descentdata is an equivalence, and(3) the inverse functor is given by (N, ϕ) 7 H 0 (s(N )).Proof. We only prove (1) and omit the proofs of (2) and (3). As R A is faithfullyflat, there exists a faithfully flat base change R R0 such that R0 A0 R0 R Ahas a section (namely take R0 A as in the proof of Lemma 3.6). Hence, usingLemma 3.8 we may assume that R A has a section, say σ : A R. Let (N, ϕ)be a descent datum relative to R A. SetM H 0 (s(N )) {n N 1 n ϕ(n 1)} NBy Lemma 3.7 it suffices to show that A R M N is an isomorphism.PTake an element n N . Writecertain ai A and xi N .Pϕ(n 1) ai xi for01By Lemma 3.2 we have n ai xPinN(becauseσ i0 δ1 id in any cosimplicialobject). Next, write ϕ(xi 1) aij yj for certain aij A and yj N . Thecocycle condition means thatXXai aij yj a i 1 xiin A R A R N . We concludeP two things fromPthis. First, by applying σ tothefirstAweconcludethatσ(ai )ϕ(xi 1) σ(ai ) xi which means thatPσ(ai )xi M.Next,byapplyingσtothemiddleA and multiplying out wePPPconclude that i ai ( j σ(aij )yj ) ai xi n. Hence by the first conclusion wePsee that A R M N is surjective.P Finally, supposeP that mi M and ai mi 0.Then we see by applying ϕ toai mi 1 thatai mi 0. In other wordsA R M N is injective and we win. 023ORemark Q3.10. Let R be a ring. Let f1 , . . . , fn R generate the unit ideal. Thering A i Rfi is a faithfully flat R-algebra. We remark that the cosimplicial ring(A/R) has the following ring in degree n:YRfi0 .fini0 ,.,inHence the results above recover Algebra, Lemmas 24.2, 24.1 and 24.5. But theresults above actually say more because of exactness in higher degrees. Namely, itimplies that Čech cohomology of quasi-coherent sheaves on affines is trivial. Thuswe get a second proof of Cohomology of Schemes, Lemma 2.1.039YRemark 3.11. Let R be a ring. Let A be a cosimplicial R-algebra. In this settinga descent datum corresponds to an cosimplicial A -module M with the property

DESCENT9that for every n, m 0 and every ϕ : [n] [m] the map M (ϕ) : Mn Mm inducesan isomorphismMn An ,A(ϕ) Am Mm .Let us call such a cosimplicial module a cartesian module. In this setting, the proofof Proposition 3.9 can be split in the following steps(1) If R R0 and R A are faithfully flat, then descent data for A/R areeffective if descent data for (R0 R A)/R0 are effective.(2) Let A be an R-algebra. Descent data for A/R correspond to cartesian(A/R) -modules.(3) If R A has a section then (A/R) is homotopy equivalent to R, theconstant cosimplicial R-algebra with value R.(4) If A B is a homotopy equivalence of cosimplicial R-algebras then thefunctor M 7 M A B induces an equivalence of categories betweencartesian A -modules and cartesian B -modules.For (1) see Lemma 3.8. Part (2) uses Lemma 3.2. Part (3) we have seen in theproof of Lemma 3.5 (it relies on Simplicial, Lemma 28.5). Moreover, part (4) is atriviality if you think about it right!4. Descent for universally injective morphisms08WENumerous constructions in algebraic geometry are made using techniques of descent,such as constructing objects over a given space by first working over a somewhatlarger space which projects down to the given space, or verifying a property of aspace or a morphism by pulling back along a covering map. The utility of such techniques is of course dependent on identifica

DESCENT 6 with notation as in Simplicial, Section 5. We first verify the two properties σ0 0 δ1 0 id and σ0 0 δ1 1 id. The first one, σ0 0 δ1 0 id, is clear from the explicit description of the morphisms above.

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