Regular readers of this blog probably know that I’m obsessed with diamonds. They can thus imagine my happiness when Peter posted an official foundational reference for diamonds a few weeks ago.
I want to use this occasion to make a remark aimed at graduate students etc. who might be wondering whether they should bother learning this stuff: in my opinion, spending time with difficult* manuscripts like the one above usually pays off in the long run. Of course, this only works if you invest a reasonable amount of time, and there’s some initial period where you’re completely befuddled, but after some months the befuddlement metamorphoses into understanding, and then you have a new set of tools in your toolkit! This shouldn’t be so surprising, though; after all, papers like this are difficult precisely because they are so rich in new ideas and tools.
Really, I’ve had this experience many times now – with the paper linked above and its precedent, with the Kedlaya-Liu “Relative p-adic Hodge theory” series, with Kato’s paper on p-adic Hodge theory and zeta functions of modular forms, etc. – and it was the same every time: for some period of months (or years) I would just read the thing for its own sake, but then at some point something in it would congeal with the rest of the swirling fragments in my head and stimulate me to an idea which never would’ve occurred to me otherwise. It’s the most fun thing in the world. Try it yourself.
*Here by “difficult” I don’t mean anything negative, but rather some combination of dense/forbidding/technical – something with a learning curve. Of course, there are plenty of papers which are difficult for bad reasons, e.g. because they’re poorly written. Don’t read them.
Just back from the 2017 Arizona Winter School on perfectoid spaces. First of all, I should say that everything was impressively well-organized, and that the lecturers did a fantastic job, especially considering the technical weight of this material. (Watch the videos if you don’t believe me.) Jared Weinstein, in particular, has an almost supernatural ability to make a lecture on some technical thing feel comforting.
Now to the jokes.
- In his opening lecture, Scholze called perfectoid spaces a “failed theory”, on account of his inability to completely settle weight-monodromy. “You see, I’m Prussian, and when a Prussian says he wants to do something, he really feels responsible for doing it.”
- Audience member: “Why are they called diamonds?”
Scholze: “[oral explanation of the picture on p. 63 of the Berkeley notes]”
Weinstein: “Also, diamonds are hard.”
- Anon.: “When you’re organizing a conference, the important thing is not to give in and be the first one who actually does stuff. Because then you’ll end up doing everything! Don’t do that! Don’t be the dumb one!”
Me: “Didn’t you organize [redacted] a couple of years ago?”
Anon.: “Yeah… It turned out that Guido Kings was the dumb one.”
- Mazur: “It just feels like the foundations of this area aren’t yet… hmm…”
Mazur: “Yes, exactly. I mean, if Grothendieck were here, he would be screaming.”
- “Do you ever need more than two legs?”
- During the hike, someone sat on a cactus.
- Finally, here is a late night cartoon of what a universal cohomology theory over might look like (no prizes for guessing who drew this):
This spectacular theorem was announced by Richard Taylor on Thursday, in a lecture at the joint meetings. Taylor credited this result and others to Allen-Calegari-Caraiani-Gee-Helm-Le Hung-Newton-Scholze-Taylor-Thorne (!), as an outcome of the (not so) secret mini-conference which took place at the IAS this fall. The key new input here is work in progress of Caraiani-Scholze on the cohomology of non-compact unitary Shimura varieties, which can be leveraged to check (at least in some cases) the most difficult hypothesis in the Calegari-Geraghty method: local-global compatibility at l=p for torsion classes.
The slides from my talk can be found here. Naturally I managed to say “diamond” a bunch of times.
Let be a proper variety over some field, and let be a vector bundle on . The functor of global sections of , i.e. the functor sending a scheme to the set , is (representable by) a nice affine -scheme, namely the scheme . Let denote the subfunctor corresponding to nowhere-vanishing sections . We’d like this subfunctor to be representable by an open subscheme. How should we prove this?
Let be the structure map. The identity map corresponds to a universal section . Let denote the zero locus of . This is a closed subset. But now we observe that the projection is proper, hence universally closed, and so is a closed subset of . One then checks directly that is the open subscheme corresponding to the open subset , so we win.
I guess this sort of thing is child’s play for an experienced algebraic geometer, and indeed it took Johan about 0.026 seconds to suggest that one should try to argue using the universal section. I only cared about the above problem, though, as a toy model for the same question in the setting of a vector bundle over a relative Fargues-Fontaine curve . In this situation, is a diamond over , cf. Theorem 22.5 here, but it turns out the above argument still works after some minor changes.
1 (from Patrick Allen) Let be a number field, and let be a cohomological cuspidal automorphic representation of some . Suppose that exists and satisfies local-global compatibility at all places, and that as predicted by Bloch-Kato. Then the following are equivalent:
a) , as predicted by Jannsen’s conjecture;
b) has the right dimension;
c) The product of restriction maps is injective.
The equivalence of a) and b) follows from Tate’s global Euler characteristic formula, but their equivalence with c) was news to me. The question of whether or not c) holds came up incidentally in my work with Jack on Venkatesh’s conjecture, so it was very pleasing to learn that it follows from Bloch-Kato + Jannsen.
2 (from Keerthi Madapusi Pera) If is semisimple and simply connected, and isotropic (i.e. contains some -split torus), then has no proper finite-index subgroups.
3 (from Stefan Patrikis) Let be as in 1) again. There are two number fields naturally associated with (besides ): the field generated by its Hecke eigenvalues, and the “reflex field” of its cohomological weight. Is there any chance that is always a subfield of ?, I asked SP. Yes, said he.
Sorry for the lack of blogging. It’s been a busy semester.
Let be an algebraically closed field, and let be a -dimensional affine variety over . According to a famous theorem of Artin (Corollaire XIV.3.5 in SGA 4 vol. 3), the etale cohomology groups vanish for any and any torsion abelian sheaf on . This is a pretty useful result.
It’s natural to ask if there’s an analogous result in rigid geometry. More precisely, fix a complete algebraically closed extension and a -dimensional affinoid rigid space over . Is it true that vanishes for (say) any and any -sheaf on for prime to ?
I spent some time trying to prove this before realizing that it fails quite badly. Indeed, there are already counterexamples in the case where is the -variable affinoid disk over . To make a counterexample in this case, let be the interior of the (closed, in the adic world) subset of defined by the inequalities for all ; more colloquially, is just the adic space associated to the open subdisk of (poly)radius . Let be the natural inclusion. I claim that is then a counterexample. This follows from the fact that is naturally isomorphic to , together with the nonvanishing of the latter group in degree .
Note that although I formulated this in the language of adic spaces, the sheaf is overconvergent, and so this example descends to the Berkovich world thanks to the material in Chapter 8 of Huber’s book.
It does seem possible, though, that Artin vanishing might hold in the rigid world if we restrict our attention to sheaves which are Zariski-constructible. As some (very) weak evidence in this direction, I managed to check that vanishes for any one-dimensional affinoid rigid space . (This is presumably well-known to experts.)
I wonder what this wild mess –
– is all about? If you’re in the Chicago area next week, come and find out!