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

16 votes

Birationally transforming a quartic elliptic curve

The method explained in Husemöller's book on elliptic curves is as follows: Take a general quartic $v^2=f_4(u)=a_ou^4+a_1u^3+a_2u^2+a_3u+a_4$, and let $$u=\frac{ax+b}{cx+d}\qquad v=\frac{ad-bc}{(cx+d) …
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2 votes

Equivalence of various definitions of arithmetic Chow groups

The general idea is that for an arithmetic variety $X$, different choices of the pair $(H^\bullet,\mathcal{C})$, where $H^\bullet$ is a cohomology theory and $\mathcal{C}$ is a complex, yield differen …
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5 votes

Hodge standard conjecture in positive characteristic

Probably too late, but... (1) Not quite. The Hodge index theorem only works for $\mathbb{C}$. To extend it to all characteristic $0$ ground fields, you need the Lefschetz principle and the comparison …
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1 vote
Accepted

Algebraic proof of Ramanujam's vanishing theorem

Perhaps this helps: Kyungho Oh, Vanishing Theorems for Singular Varieties, (Proceedings of the American Mathematical Society).
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1 vote

Kummer extension of Galois modules

I'm not sure if this is what you are looking for, but consider the Tate curve $E_q$ on your local field $k$. It sits on the exact sequence of $G_k$ modules. $$0 \to \mu_n \to E_q[n] \to \mathbb{Z}/n\ …
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3 votes
Accepted

Why is $\mathbb{Q}_p(p^{1/p^\infty})$ a complete topological field?

This was alredy answered in the comments, it is the $p$-adic completion of $\mathbb{Q}_p(p^{1/p^\infty})$ that is a perfectoid field. But here is a reference for completeness Matthias Wulkau, Revie …
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11 votes

When did "Betti cohomology" come to be used the way it is today? (and how is it used)

Some of this stuff is explained in much detail in this paper: Charles Weibel, History of Homological Algebra (1999) In particular, after reviewing the work of Riemman and Betti, it says: Insp …
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14 votes
0 answers
1k views

Status of the "anabelian dream" ($\mathrm{dim} \leq 1$)

The anabelian conjectures for small dimensions have been known for quite some time. In full generality the results are: Dimension 0. Finitely generated fields are anabelian (Pop) Dimension 1. Hyperb …
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30 votes
2 answers
5k views

Grothendieck's "La longue Marche à travers la théorie de Galois"

It seems that Grothendieck's familly has given permission for the distribution of his unpublished works, so I hope it is ok to ask this. Is there any way to obtain a copy (online or not) of "La lo …
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19 votes
Accepted

Grothendieck's "La longue Marche à travers la théorie de Galois"

All the manuscripts have been finally made avaible by Montpellier university. You can find all of them here, avaible in pdf. The items related to “La longue Marche" are: La "Longue Marche" à traver …
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30 votes

Progress on the standard conjectures on algebraic cycles

For future references. Feel free to edit to include new cases, or any improvements. As for 2015, the standard conjectures on algebraic cycles is unconditionally (at lest) known for $X$: Lefschetz st …
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12 votes

$\zeta(n)$ as a mixed Tate motive

Partially inspired by an unpublished work of Wojtkoviak for $\zeta (3)$, all of this follows from this result by Deligne: Theorem. $\pi_1(\mathbb{P}^1-\{0,1,\infty\})$ is a smooth mixed Tate motive o …
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4 votes
Accepted

Motivation for Hirzebruch-Jung Modified Euclidean Algorithm

I think the motivation for the Hirzebruch-Jung algorithm is not the algorithm itself, but the fact the it yields directly a very interesting continued fraction expansion. It is those, now called Hirze …
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9 votes

Which elliptic curves over totally real fields are modular these days?

Elliptic curves over real quadratic fields were proven to be modular very recently by Freitas, Le Hung and Siksek: Nuno Freitas, Bao Le Hung, Samir Siksek, Elliptic Curves over Real Quadratic Fields …
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9 votes

Crystalline realization of mixed Tate motives

Apparently this issue was worked out shortly after, during Go Yamashita's stay at the IHES in 2006, and after some discussion with Deligne. For any Tate motive $M$ unramified at $v$, its crystalline …
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