Tag: Harris

The dangers of naming things after people

I spent part of last weekend reading Alice Silverberg’s blog, which is simultaneously depressing and hilarious. Everyone should read it, but you probably won’t enjoy reading it unless you enjoy Coen brothers movies or the short stories of Kafka. Anyway, the following thoughts have been in my head for a few months, but I decided to record them here after reading this.

The cohomology of non-basic local Shimura varieties is described by the “Harris-Viehmann conjecture”, which is formally stated as Conjecture 8.4 in Rapoport-Viehmann’s paper. This story started with a daring and beautiful conjecture of Harris (conjecture 5.2 here), whose formulation however turned out to be slightly incorrect in general, cf. Example 8.3 in RV. The conjecture was then modified by Viehmann, and Rapoport named this modified conjecture the Harris-Viehmann conjecture (footnote 5 in RV).

Unfortunately, Conjecture 8.4 in RV is still not correct as stated: the Weil group action on the summands appearing on the right-hand side needs to be modified by certain half-integral Tate twists. As far as I know, Alexander Bertoloni-Meli is the only person who has publicly pointed out the need for this modification, and Conjecture 3.2.1 in his very cool paper is the only correct formulation of the Harris-Viehmann conjecture in print. 

Since the need for these Tate twists was overlooked by a lot of very smart people, it only seems fair to me that Alexander should get credit for his contribution here. The obvious way to do this would be to refer to the Harris–Viehmann–Bertoloni-Meli conjecture, or the Bertoloni-Meli–Harris–Viehmann conjecture. You could pick the second option if you’re a stickler for alphabetical name orders in mathematics, or the first option if you feel (as I do) that Harris’s contribution here deserves priority.

But it gets even worse, because Harris also formulated another conjecture along similar lines (conjecture 5.4 in his article linked above), which has gotten somewhat less attention but which is nevertheless extremely interesting.* It turns out that one can formulate a unified conjecture which encompasses both Harris’s conjecture 5.4 and the Harris–Viehmann–Bertoloni-Meli conjecture. What should it be called? The Harris–Viehmann–Bertoloni-Meli—Harris conjecture? I guess not.

*Here’s a comment from MH: “I was (and am) much more attached to this conjecture than to the one that is called the Harris-Viehmann conjecture, because it required some work to find the right formalism (the parabolics that transfer between inner forms), whereas the other conjecture (independently of the incorrect formulation in my paper) was just the obvious extension of Boyer’s result.”

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Better than excellent

MH once pointed out the “linguistic trap” Grothendieck created when he defined the notion of an excellent ring: “Suppose somebody finds an even better class of rings? Then what?”

It turns out there IS an even better class of rings/schemes, which occurs naturally in some contexts.

Definition. A scheme X is marvelous if it is Noetherian and excellent, and if \dim \mathcal{O}_{Y,y} = \dim Y for every irreducible component Y \subset X and every closed point y \in Y. A ring A is marvelous if \mathrm{Spec}(A) is marvelous.

You can easily check that any marvelous scheme is finite-dimensional. Moreover, it turns out that a Noetherian quasi-excellent scheme is marvelous if and only if the function x \in |X| \mapsto \dim \overline{ \{ x \} } is a true dimension function for X (in a certain technical sense). This function is of course the most naive and clean possibility for a dimension function on any given scheme, but it doesn’t always have the right properties.

Unfortunately, marvelous schemes are so marvelous that, unlike excellent schemes, they aren’t stable under many natural operations, not even under passing to an open subscheme! In fact, X is marvelous if it is covered by marvelous open affines, but the converse fails. You can check that a scheme as simple as \mathrm{Spec}\mathbf{Z}_p[x] isn’t marvelous, even though \mathrm{Spec}\mathbf{Z}_p is marvelous. So regular excellent schemes aren’t always marvelous, and adjoining a polynomial variable can kill marvelousity. I briefly entertained the hope that any Jacobson excellent scheme is marvelous, but this fails too (the scheme S considered in EGAIV3 (10.7.3) is a counterexample).

It’s not all bad news, though:

  1. anything of finite type over \mathbf{Z} or a field is marvelous,
  2. any excellent local ring is marvelous,
  3. any ring of finite type over an affinoid K-algebra in the sense of rigid geometry is marvelous,
  4. any scheme proper over a marvelous scheme is marvelous; more generally, if X is marvelous and f: Y \to X is a finite type morphism which sends closed points to closed points, then Y is marvelous,
  5. if A is a marvelous domain, then the dimension formula holds: \dim (A/\mathfrak{p}) + \mathrm{ht}\,\mathfrak{p} = \dim A for all prime ideals \mathfrak{p} \subset A. (Recall that the dimension formula can fail, even for excellent regular domains.)

You might be wondering why I would care about such a stupid and delicate property. The reason is the following. Fix any marvelous scheme X and any n invertible on X. Then there is a canonical potential dualizing complex \omega_{X} \in D^{b}_{c}(X,\mathbf{Z}/n) (in the sense of Gabber) which restricts to \mathbf{Z}/n[2\dim ](\dim) on the regular locus of X. Here \dim is the (locally constant) dimension of the regular locus, so this numerology is the same as in the case of varieties. Moreover, for any prime \ell invertible on X, there is a good theory of \ell-adic perverse sheaves on X with the same numerology as in the case of varieties; in particular, the IC complex restricts to \mathbf{Q}_{\ell}[\dim] on the regular locus. (See sections 2.2 and 2.4 of Morel’s paper for more. Note in particular the hypothesis on X in the first sentence of section 2.2: it is exactly the condition that X is marvelous.) This discussion all applies, in particular, when X=\mathrm{Spec}(A) for any K-affinoid ring A. This turns out to be an important ingredient in my forthcoming paper with Bhargav…

(One more comment: Most real-life examples of marvelous schemes, e.g. examples 1. and 3. above, are also Jacobson. It might be more reasonable to consider the class of marvelous Jacobson schemes, because these are permanent under finite type maps. But on the other hand we lose excellent local rings when we do this.)