# Fall roundup

Apologies for the lack of blogging. This has been an unusually busy fall.

• My student Linus Hamann has a website! Please go there and check out his beautiful preprints, especially his paper on comparing local Langlands correspondences for GSp4.
• There have been a lot of great papers this year, but I was especially struck by these gorgeous ideas from Teruhisa Koshikawa. Readers might recall that the seminal Caraiani-Scholze papers contain a fun part (p-adic geometry of Shimura varieties and their Hodge-Tate fibers, semiperversity of Hodge-Tate pushforwards) and a not fun part (arguments with the twisted stable trace formula and Shin’s stable trace formula for Igusa varieties). Koshikawa completely eliminates the not fun part, replacing it with an extremely clever use of the Fargues-Scholze machinery. Even in the setting of the CS papers, Koshikawa’s main theorem is stronger; moreover, his technique opens the door to a wide generalization of the CS vanishing results beyond the specific unitary Shimura varieties they treated. (Note for ambitious readers: The problem of working out these generalizations has already been “taken” by specific people.)
• Eagle-eyed readers of H.-Kaletha-Weinstein might’ve noticed that the entire paper depends crucially on a non-existent preprint cited as [GHW]. As discussed in a previous post, the point of GHW is to construct the functor $Rf_!$ in etale cohomology for certain stacky maps of Artin v-stacks, by adapting some machinery of Liu-Zheng which they built to solve the analogous problem in the setting of Artin stacks. Since the above-mentioned papers of Hamann and Koshikawa both depend directly on HKW, and thus indirectly on GHW, I’ve felt some increased pressure recently* to actually produce this paper!
However, I think this pressure helped push me past the final points of confusion in this project, and I’m pleased to report that after nearly 4 years of struggle, the details of GHW have finally come together. I’m cautiously optimistic that the paper will be publicly available within a few months. The arguments are an infernal mixture of delicate p-adic geometry and general $\infty$-categorical constructions. Actually, this is the most intense and frustrating project I’ve ever worked on. It will be good to finish it.
• As always, David Roberts offers a voice of clarity against the nonsense burbling out from the IUT cultists.

*Both from myself and from the referee for HKW.

## 3 thoughts on “Fall roundup”

1. Really? says:

Wow at least 3 papers crucially depending on a forthcoming work of an author that is admittedly still confused about it: I am glad I don’t work in your area!

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1. Well it’s common that paper A depends on something that will go into paper B, and details of that part has been sorted out, but not some other part. Or one hasn’t found the best version of theorems to be proved in paper B. I wonder if there’s an area in math where such a thing doesn’t happen?

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1. Really? says:

Yeah I misunderstood the blog post
And understood these papers were conditional on the main result of this forthcoming paper. My bad!

Yeah what you write is quite common.

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