Dilatations

We clarify the discussion in this post resulting in a generalization of a result of Mike Artin.

Let X be a Noetherian algebraic space. Let T ⊂ X be a closed subspace. Let us denote X/T the formal completion of X along T (Tag 0AIX). Let W —> X/T be a rig-etale morphism of formal algebraic spaces, which means that

1. W —> X/T is representable by algebraic spaces, i.e., it is an adic morphism of formal algebraic spaces Tag 0AQ2),
2. W —> X/T is locally of finite type, i.e., it is etale locally on affine formal algebraic pieces given by a continuous ring map A —> B which is topologically of finite type Tag 0ALL),
3. these ring maps A —> B have a completed cotangent complex whose cohomology groups are annihilated by an ideal of definition of A, for more details see Tag 0ALP and Tag 0AQK.

These three conditions correspond to condition (i) of Definition 1.7 of Artin’s paper “Formal Moduli: II”. The first result is that

Given W —> X/T rig-etale there exists a morphism of algebraic spaces Y —> X which is an isomorphism over X – T and whose completion Y/T —> X/T is isomorphic to W —> X/T.

In fact, Theorem 0ARB tells us we obtain an equivalence of categories.

This theorem does not often produce separated morphisms Y —> X if we start with a random W —> X/T. A typical thing that happens can be seen by starting with X equal to the affine line, T = {0} and W two copies of the completion of X at 0. Then the resulting Y is the affine line with 0 doubled.

Thus to get Artin’s theorem on dilatations we need to impose conditions on W —> X/T guaranteeing that Y —> X is separated or even proper. To do this we will use the notion of a rig-surjective morphism W’ —> W of locally Noetherian formal algebraic spaces W, W’ defined by requiring adic morphisms Spf(R) —> W with R a cdvr to lift to Spf(R’) —> W’ for some extension of cdvr R ⊂ R’ (Tag 0AQP). Let’s say an adic morphism W —> W’ of locally Noetherian formal algebraic spaces is a rig-monomorphism if the diagonal morphism is rig-surjective. In this language the conditions (ii) and (iii) from Definition 1.7 of Artin’s paper have the following interpretations:

1. If W —> X/T as above is separated and a rig-monomorphism, then Y —> X is separated (Tag 0ARW),
2. If W —> X/T as above is proper, a rig-monomorphism, and rig-surjective, then Y —> X is proper (Tag 0ARX).

The second statement recovers exactly Artin’s theorem on dilatations.

One typically applies the result to construct modifications Y —> X (Tag 0AD7) by taking the complete local ring A of X at a closed point x and setting W equal to the formal completion of the blow up of A at an ideal I ⊂ A which is locally principal on the puctured spectrum of A. Here the funny situation occurs that we can first read Tag 0ARX backwards over Spec(A) to conclude that W —> X/x has the required properties and then forwards to conclude that Y —> X is proper. In other situations it may not be that easy to verify the assumptions needed for the application of the theorem and it would behoove us to prove a few lemmas that help with this task.

Spaces are fpqc sheaves

Although already mentioned in the previous post I’d like to point out once again that we have, thanks to Ofer Gabber, now a proof that algebraic spaces satisfy the sheaf condition for fpqc coverings, answering one of the questions in this post. Also thanks go to Bhargav Bhatt who sent me an email (on 9/13/13) explaining Gabber’s argument. With small comments/corrections/clarifications added over the weekend, I hope most of the kinks I introduced have been ironed out.

To read more, start here. As usual comments and suggestions for improvements are welcome. Enjoy!

Update

Since the last update of June 2013 we have added the following material:

1. Algebraicity of stack of coherent sheaves Tag 09DS
2. Epp’s Theorem Tag 09F9 and Tag 09II
3. Reduced fibre theorem Tag 09IL
4. New chapter on fields Tag 09FA
5. Perfect generator for DQCoh(X) for schemes Tag 09IS
6. Perfect generator for DQCoh(X) for algebraic spaces Tag 09IY
7. New chapter on differential graded algebra Tag 09JD
8. Section on Noetherian schemes of dimension 1 Tag 09N7
9. Finite set of codimension 1 points on separated scheme are in an affine Tag 09NN
10. Section on curves Tag 0A22
11. Versions of Avramov’s result Tag 09Q7 and Tag 09QF
12. Obtaining triangulated categories Tag 09P5 written by John Yu and Yifei Zhao
13. Rickard’s theorem Tag 09S5
14. Section on compact objects Tag 09SM
15. Section on generators in triangulated categories Tag 09SI
16. DQCoh = D(QCoh) for Noetherian schemes Tag 09TI
17. DQCoh = D(QCoh) for Noetherian spaces Tag 09TH
18. Final exam commutative algebra Fall 2013 Tag 09TV
19. Lots of stuff about pseudo-coherence
20. Cohomology of locally compact spaces Tag 09V0
21. Proper base change in topology 09V4
22. New chapter on simplicial spaces Tag 09VI
23. Cohomological descent for proper hypercoverings in topology Tag 09XA
24. Section on henselian pairs Tag 09XD
25. Finite cover by a scheme Tag 0ACX
26. Partitions and stratifications Tag 09XY
27. Bunch of stuff about limits of sites and spaces and cohomology
28. Gabber’s result on Henselian pairs Tag 09Z8
29. Theorem of formal functiosns via derived completion Tag 0A0H
30. Lots of edits to the chapter on etale cohomology culminating in a proof of the proper base change theorem Tag 095S
31. Cohomological dim <= Krull dim for spectral spaces Tag 0A3C
32. Equivalence beteen torsion and derived complete modules Tag 0A6V
33. Local duality a la Grothendieck Tag 0A81
34. Brown representability for triangulated categories Tag 0A8E
35. Final exam algebraic geometry Spring 2014 Tag 0AAL
36. Bunch of stuff about twisted inverse image, dualizing complexes, upper shriek functors
37. New mostly empty chapter on resolution of surfaces Tag 0ADW
38. Algebraic spaces are schemes in codimension 1 Tag 0ADD
39. Extremely careful discussion of points on fibres of morphisms of algebraic fibres and when this implies the morphism is quasi-finite at the points Tag 0AC0
40. Finite groupoids Tag 0AB8
41. fppf descent data for spaces over spaces are effective Tag 0ADV
42. New chapter on pushouts of algebraic spaces Tag 0AHT including a discussion of something called formal glueing which should really have another name
43. Regular local rings are UFDs Tag 0AFW
44. Approximation for henselian pairs Tag 0AH4

This brings us up to July 1 of this year. At this point I decided to work out what happens with dilatations in Artin’s paper “Formal Moduli II”. In fact, I wrote a blog post trying to figure out what could be true. Anyway, it turns out that Artin’s theorem on dilatations holds without assuming the base Noetherian algebraic space is excellent and in the proof we don’t need to use Artin’s criteria. See Theorem Tag 0ARB and combined this with Lemma Tag 0ARX to obtain Artin’s theorem — I hope to discuss what these results mean in more detail in a later blog post. Here are some related and not so related changes to the Stacks project.

1. A chapter on formal algebraic spaces. Our approach is somehow a combination of all approaches in the literature: EGA, McQuillan, Yasuda, Beilinson-Drinfeld, Abbes, Fujiwara-Kato, and Knutson. Namely, we define a formal algebraic space as an fppf sheaf on (Sch) which \’etale locally looks like an Ind-scheme whose transition morphisms are thickenings. For the Stacks project this approach turns out to be exactly the right one and many of the earlier results on algebraic spaces can be brought to bear to get ahead fairly quickly.
2. A followup chapter discussing the notion of rig-etale (homo)morphisms ending with a proof of the algebraization theorem mentioned above. These rig-etale maps are used by Artin in his paper mentioned above to describe formal modifications and these (as well as the similarly defined rig-smooth ring maps) are the ring maps studied in Elkik’s famous paper.
3. Adjoint functor theorem Tag 0AHM
4. Remarkable result by Yasuda that every qcqs formal algebraic space is a filtered colimit of algebraic spaces along thickenings Tag 0AJD
5. Non flat completions Tag 0AL8
6. QCoh not abelian for formal algebraic spaces Tag 0ALF
7. Ofer Gabber answered completely this question see Tag 0APW
8. Algebraic spaces are fpqc sheaves by Ofer Gabber Tag 03W8

Enjoy!