OK I’m too lazy to convert this into a proper blog post, but Proposition 0.4 here is something I needed recently, which others might find useful. Lemma 0.2 might also come in handy. Please ask in the comments if you’d like more details!
I’m going to do this two-songs-a-day thing until I get bored or things start to return to normal.
Today, two songs from the incomparable guitarist. MH is already a fan – now you can be too!
If you need a few moments of distraction right now, you could do a lot worse than either of these songs, the second of which is probably one of the funniest tunes in the whole jazz canon. Enjoy!
Anyone who writes “in the sequel” or “rôle” in their papers should be fined a huge amount of money.* It’s 2020, folks.
*The money will be spent on higher categories for the needy.
In p-adic geometry, what should it mean for a morphism to be a Zariski-closed immersion? For locally Noetherian adic spaces, the usual notion of a closed immersion of locally ringed spaces works just fine. For general analytic adic spaces, though, one quickly runs into annoying foundational issues. The issue is roughly as follows. Let be an (analytic) affinoid adic space. We can certainly define a reasonable notion of Zariski-closed subset, just by following our nose: a subset should be Zariski-closed if there is an ideal such that iff . These are exactly the subsets obtained by pulling back closed subsets of along the natural map . The problem, however, is that such a need not come from an actual closed immersion of an affinoid adic space into , because the quotient could just be some junky non-sheafy ring, and maybe there’s no canonical tweak (like replacing by its closure, or replacing by its uniform completion, or…) which will make it sheafy. And even if we can tweak to make it sheafy, how do we know that is still surjective after going to some rational subset ? You get the picture.
Perhaps surprisingly, the situation for affinoid perfectoid spaces is a lot better. In particular, if is a perfectoid Tate-Huber pair, there are canonical bijections (satisfying some obvious compatibilities) between
1) closed subsets of ,
2) Zariski-closed subsets of ,
3) (isomorphism classes of) maps of Tate-Huber pairs where is a perfectoid Tate ring, is surjective, and is the integral closure of the image of in .
We’ve already discussed the bijection 1) <–> 2). For 3) –> 1) or 2), just send to the closed subset cut out by the ideal . The miracle is the association 2) –> 3), which holds by an amazing theorem of Bhatt: if is a closed ideal in a perfectoid Tate ring , then the uniform completion of is perfectoid and the natural map is surjective, cf. Theorem 2.9.12 in Kedlaya’s notes here. Moreover, the map remains surjective after rational localization on . In particular, if is a Zariski-closed subset, then 2) –> 3) gives an honest closed immersion of locally ringed spaces, and maps homeomorphically onto .
The point of all this is that Zariski-closed immersions of affinoid perfectoid spaces behave as well as one could ever dream (with one caveat, which I’ll get to later). The following definition then suggests itself.
Definition. A map of small v-stacks is a Zariski-closed immersion if for any affinoid perfectoid space with a map , the base change is a Zariski-closed immersion of affinoid perfectoid spaces.
Now of course we’re free to make any definition we want in mathematics, but if it doesn’t capture some essential idea or experimentally observed phenomenon, then who cares? Let me now give some evidence that this definition passes this test.
Example 0. The property of being a Zariski-closed immersion is preserved under composition and base change. If is a Zariski-closed immersion and is (a small v-sheaf, a diamond, a locally spatial diamond, qc or qs or separated or partially proper over a base ), then so is .
Example 1. Let be a closed immersion of locally Noetherian adic spaces. If is affinoid (so is too), then the map of diamonds is a Zariski-closed immersion. This is easy.
Example 2. Let be a closed immersion of locally Noetherian adic spaces again, but now assume that is the analytification of a closed immersion of quasiprojective varieties. Then is a Zariski-closed immersion. For this, we can use the assumption on to choose a vector bundle on together with a surjection . Then for any map from an affinoid perfectoid, the pullback (in the usual sense of ringed spaces) is a vector bundle on , hence generated by finitely many global sections by Kedlaya-Liu. The images of along the natural map generate an ideal, and the associated closed immersion of affinoid perfectoids represents the fiber product . (Hat tip to PS for suggesting this vector bundle trick.)
Example 3. Let be a minimally compactified Hodge-type Shimura variety with infinite level at . Then the boundary is a Zariski-closed immersion, and so is the diagonal . (These both reduce to the previous example, using a small limit argument in the second case.) In particular, if are any open affinoid perfectoid subsets, then is also affinoid perfectoid. This small observation plays a non-negligible role in my forthcoming paper with Christian Johansson, where (among other things) we prove that any minimally compactified Shimura variety of pre-abelian type with infinite level at is perfectoid.
Example 4. Fix a perfectoid base field of characteristic zero. Then the inclusions are Zariski-closed immersions of (ind-)diamonds over . This can be proved by induction on , and the base case reduces to the fact that the inclusion is the pullback of along . (To make the induction work, you need to pick an element generating .)
Example 5. Fix a complete algebraically closed extension . Fix a reductive group and a geometric conjugacy class of -valued cocharacters . Then is a Zariski-closed immersion. Also, if , then is a Zariski-closed immersion. These claims can be reduced to the case , which in turn reduces to Example 4 by some trickery.
Example 6. Fix a complete algebraically closed nonarchimedean field of residue characteristic , and let be any injective map of coherent sheaves on the Fargues-Fontaine curve . Then the associated map of Banach-Colmez spaces is a Zariski-closed immersion. This can also be reduced to Example 4.
Let me end with some caveats. First of all, I wasn’t able to prove that if is a closed immersion of reductive groups, the induced map is a Zariski-closed immersion, although it is surely true. The problem here is (roughly) that an -torsor over some affinoid perfectoid can only be reduced to a -torsor locally in the analytic topology on , and we then run into the following open question:
Question. Is the property of being Zariski-closed local for the analytic topology? More precisely, if is affinoid perfectoid with a covering by rational subsets , and is a closed subset such that is Zariski-closed in for all , is Zariski-closed?
There are also naturally occurring closed things which probably aren’t Zariski-closed immersions. For instance, I don’t think the map of Banach-Colmez spaces is a Zariski-closed immersion, because then pulling back would imply that is a Zariski-closed immersion, which seems extremely unlikely to me. (But I didn’t manage to disprove it! Actually, can one give an explicit example of an affinoid perfectoid and a closed subset such that maps isomorphically to the completed residue field at every point in and such that is NOT Zariski-closed? Surely such examples exist.) I also don’t think (closures of) Newton strata in flag varieties are Zariski-closed immersions – they are just too weird.
I also wasn’t able to settle the following compatibility (but admittedly I didn’t try very hard).
Question. Let be a monomorphism of locally Noetherian adic spaces. If is a Zariski-closed immersion, is actually a closed immersion?
Happy new year!