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Piotr Achinger's user avatar
Piotr Achinger's user avatar
Piotr Achinger's user avatar
Piotr Achinger
  • Member for 14 years, 10 months
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Topological interpretation of the existence part of the valuative criterion for properness
@AriyanJavanpeykar I agree this should be true if $U$ is a Zariski open. Does it hold for all opens? Take $U$ to be the open unit disc inside $X=\mathbf{P}^1$. Then every map $f:D \to X$ sending $D^*$ into $U$ sends the origin into $U$ (since the image of $D\to X$ is open (if $f$ is not constant) and contained in $U\cup \{f(0)\}$). But $U$ is not compact.
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Topological interpretation of the existence part of the valuative criterion for properness
Does the valuative criterion work as stated for complex spaces? I’m afraid that essential singularities will ruin the analogy with algebraic geometry. For example, consider the map $\mathbb{D}^*\to \mathbf{P}^1$ given by $(e^{1/z}:1)$. It doesn’t extend across the puncture since the function is not meromorphic at zero.
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Length of analytic holonomic D-modules
However, things should be OK if you consider only $D_X$-modules which are "locally constant" on a given Zariski stratification of $X$.
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Length of analytic holonomic D-modules
Not every holonomic $D_U$-module extends to $X$: let $X = \mathbf{C}$, $U=\mathbf{C}\setminus 0$, let $Z\subseteq U$ be a discrete subset with point of accumulation $0$. Then the skyscraper $D_U$-module supported on $Z$ ($i_+ \mathcal{O}_Z$ where $i\colon Z\to U$ is the inclusion) is holonomic but does not extend to a coherent $D_X$-module.
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Triple comparison of cohomology in algebraic geometry
Keerthi's comment is spot on and IMO should be posted as an answer.
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Triple comparison of cohomology in algebraic geometry
Intuition, perhaps misguided: Aren't you secretly choosing two embeddings $\overline{\mathbf{Q}}\hookrightarrow \mathbf{C}$ and $\overline{\mathbf{Q}}\hookrightarrow \overline{\mathbf{Q}}_p$ on which these isomorphisms depend? If you then change one but not the other, wouldn't your composition change?
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Roadmap for p-adic geometry
Bosch’s book “Lectures on Formal and Rigid Geometry” is a good start. And Kedlaya’s lectures in the AWS volume on perfectoid spaces.
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Homeomorphic endomorphism of schemes inducing equivalence of sheaves
Even worse, $F$ does not a priori act on the cohomology of some sheaf $\mathcal{F}$ (unless e.g. $\mathcal{F}$ is constant). Rather, it induces a map $F^* \colon H^*(X, \mathcal{F}) \to H^*(X, F^* \mathcal{F})$ between two different groups. To compare it with the identity, you need an "equivariant structure" i.e. some choice of isomorphism $F^*\mathcal{F}\to \mathcal{F}$. In case of the Frobenius, such a canonical structure is described in the paragraph preceding the result in SGA5 you cite. P.S. $F$ is affine since the preimage of an affine open $U$ is $U$, which is affine.
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Homeomorphic endomorphism of schemes inducing equivalence of sheaves
I disagree with the statement in the second paragraph: consider the case when $F$ is an isomorphism. Certainly not every automorphism induces the identity on cohomology! Did you mean to assume that $F$ is the identity on the underlying space? But then, the case of the Zariski topology is trivial.
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Is being $S_2$ an étale-local property?
By flatness of $Y\to X$, a regular sequence in a local ring on $X$ produces a regular sequence in a local ring on $Y$.
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Étale- or fppf-crystalline sites
Fppf-crystalline could be tricky as typically you use smoothness a lot, and smoothness is not fppf local
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Étale- or fppf-crystalline sites
I imagine you already checked in Olsson’s Asterisque on crystalline cohomology of stacks?
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Unramified lisse $\overline{\mathbb{Q}}_{\ell}$-sheaves
Is $X$ assumed to be regular (at least at every point of $D$)?
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Sheaves which are locally free on subschemes of dimension zero
A possible source of lots of such sheaves: if $X$ is over $\mathbf{F}_p$ and $F\colon X\to X$ is the Frobenius, then for any given $n\geq 0$ there is an $e\geq 0$ such that for every coherent sheaf $\mathcal{F}$ on $X$, the $e$-th pullback $(F^e)^* \mathcal{F}$ has this property.
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