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1 vote
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Connecting homomorphism in non-abelian cohomology

$\newcommand{\diag}{{\rm diag}} \newcommand{\sH}{{\mathcal H}} \newcommand{\R}{{\mathbb R}} \newcommand{\HH}{\sf H} \newcommand{\V}{{\sf V}} \newcommand{\B}{{\sf B}} \newcommand{\C}{{\Bbb C}} $No, th …
Mikhail Borovoi's user avatar
3 votes

Minimal parabolic subgroups are $G(k)$-conjugate: a cohomological interpretation?

For simplicity we write $G$ for $G^{\rm der}$ and $M_0$ for $M_0^{\rm der}$, then $G$ is a connected reductive group, and $M_0$ is a Levi subgroup of a minimal parabolic $P_0$ of $G$. Let $$\xi\in{\rm …
Mikhail Borovoi's user avatar
6 votes
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Non-abelian Ext functor and non-abelian $H^2$

EDITED, taking into account the comments of Donu Arapura. As JLA wrote, a homomorphism $f\colon G\to N$ gives an extension \begin{equation}\label{e:E} 1\to K\to E\to G\to 1.\tag{E} \end{equation} This …
Mikhail Borovoi's user avatar
5 votes

Second nonabelian group cohomology: cocycles vs. gerbes

Nonabelian $H^2$ in Galois cohomology can be defined in terms of: (1) cocycles, (2) extensions, (3) gerbes. The relations between these three definitions are described in Section 2.2 of Le principe de …
Mikhail Borovoi's user avatar