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Algebraic varieties, stacks, sheaves, schemes, moduli spaces, complex geometry, quantum cohomology.

1 vote

Direct image for crystals?

If $f$ is quasi-finite, you can follow chapter IV of Berthelot's thesis (Springer Lecture notes 407). Otherwise, you will need to enter the derived world, much like in the case of D-modules. Gaitsgo …
S. Carnahan's user avatar
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1 vote

Universal etale covering, reference request

It's not clear what you mean by "exactly same with" in your theorem, but even if we generously interpret it to mean that the map $\bar{Y} \to \bar{X}$ factors through $\tilde{\bar{X}}$, the claim does …
S. Carnahan's user avatar
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3 votes
Accepted

The stable general linear group in algebraic geometry

It's an ind-affine ind-algebraic group - the usual choice of increasing union gives a diagram of closed embeddings of affine algebraic groups. It is a sheaf in any topology where $GL_n$ is a sheaf, e …
S. Carnahan's user avatar
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7 votes
Accepted

Fundamental groups of $\mathcal{M}_{0,n}$

Over an algebraically closed field of characteristic zero, a choice of complex analytification gives you an equivalence between finite étale covers of the moduli scheme $\mathcal{M}_{0,n}$ and finite …
S. Carnahan's user avatar
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5 votes
Accepted

are these functors exact?

I assume you're asking about quasicoherent sheaves. Since the inclusion of $U$ into $X$ is an open immersion, $j_!$ is exact. You can find this in Tag 03DJ. Since $j$ is an affine morphism, $j_*$ i …
S. Carnahan's user avatar
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2 votes
Accepted

Formal base change properties of group schemes

To show that the scheme isomorphism is an isomorphism of group schemes, it suffices to check that the multiplication maps coincide, i.e., $$m_{(X \times_S Y)_{S'}} = \phi^{-1} \circ (m_{X_{S'}} \times …
S. Carnahan's user avatar
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3 votes

How should I think about this scheme constructed from a line bundle

It is common to use the name "the $\mathbb{G}_m$-torsor associated to $\mathscr{L}$", and it is concisely written as $L = \underline{\operatorname{Isom}}_X(\mathscr{O}_X, \mathscr{L})$. However, an a …
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4 votes

Surjectivity of tangent spaces induced by smooth morphism

Willy Liu answered this in a comment. By EGA 4v4 Corollary 17.11.4, a necessary and sufficient condition that $X \to Y$ be smooth at $x \in X$ is that there be an open neighborhood $U \subset X$ of $ …
2 votes

Is there a concrete description of relative jet spaces that invokes neither internal Hom nor...

For your first question, I think the answer depends on whether you think representability means "in schemes" or "in algebraic spaces". For schemes, you really want a projective morphism $Y \to S$ for …
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3 votes

Tannakian fundamental group for finitely linear representation of group

Following Niels's suggestion, I'm turning my comments into an answer. The first point I want to make is the observation that if $G$ is an infinite group, then the constant group scheme $G_k$ is not a …
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6 votes

Is there any value in studying divisors with coefficients in a ring R?

If you view a $\mathbb{Z}$-valued Cartier divisor (on say, an integral separated scheme X) as a $\mathbb{G}\_m$-torsor on X with generic trivialization, then for any torus T with character group $X^\a …
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3 votes
Accepted

When does a projective morphism give an etale morphism (into affine space)? (Finite field) (...

This paper by Kedlaya might be what you want, since it contains some rearrangement of the words you used, but I can't really tell from the question. If you want a proper F-scheme to have an etale map …
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0 votes
Accepted

Trivialisation of Mobius Line Bundle

There appears to be some confusion in the question: The circle $S^1$ is not a complex manifold, so it does not admit a meaningful notion of holomorphic line bundle. If you try to construct a complex …
S. Carnahan's user avatar
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4 votes

$D_X$ algebras, $D_X$ schemes, connections

Keerthi Madapusi Sampath already answered the question in the comments, so I'll just add some remarks. The definitions in question are given in section 2.3.1, on page 80 of Beilinson and Drinfeld's …
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4 votes
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map defined by element of function field of a variety

The question was answered in the comments, so I'm adding this to knock the question off the "unanswered" list. The literal answer to your first question is "Yes if and only if $n \leq 1$".

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