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Sep
10
awarded  Nice Answer
Sep
9
answered Is the category of commutative group schemes abelian?
Sep
1
comment Is there a classification of embeddings of SL_2 into SP_6 as algebraic groups over Q and R respectively?
Assume the base field has characteristic zero. The category of representations of a semisimple Lie algebra is Tannakian, and the algebraic group attached to this category is the simply connected algebraic group with the given Lie algebra. A homomorphism of Lie algebras defines a tensor functor of the Tannakian categories, and hence a homomorphism of the corresponding simply connected algebraic groups.
Aug
31
comment Why aren't there more classifying spaces in number theory?
Continued: It was then found that $H^{1}$ coincided with the group of crossed homomorphisms modulo principal crossed homomorphisms, and $H^{2}$ with the group of equivalence classes of "factor sets", which had been introduced much earlier (e.g., I. Schur, \"{U}ber die Darstellung der endlichen$\ldots$ , 1904; O. Schreier, \"{U}ber die Erweiterungen von Gruppen, 1926; R. Brauer, \"{U}ber Zusammenh\"{a}nge$\ldots$ , 1926). For more on the history, see MacLane 1978 (Origins of the cohomology of groups. Enseign. Math. (2) 24 (1978), no. 1-2, 1--29. MR0497280).
Aug
31
comment Why aren't there more classifying spaces in number theory?
From my Class Field Theory Notes p86. In the mid-1930s, Hurewicz showed that the homology groups of an "aspherical space" $X$ depend only on the fundamental group $\pi$ of the space. Thus one could think of the homology groups $H_{r}(X,\mathbb{Z})$ of the space as being the homology groups $H_{r}(\pi,\mathbb{Z})$ of the group $\pi$. It was only in the mid-1940s that Hopf, Eckmann, Eilenberg, MacLane, Freudenthal and others gave purely algebraic definitions of the homology and cohomology groups of a group $G$.
Aug
23
awarded  Nice Answer
Aug
23
awarded  Necromancer
Aug
23
answered Where can we find Deligne's paper “ Theorie de Hodge I”?
Aug
16
comment Inaccessible cardinals and Andrew Wiles's proof
The Stack Project develops a huge amount of Grothendieck style mathematics, including a lot of etale cohomology, using only ZFC (specifically, NOT using universes). If anyone has any doubt that this can be done, I suggest that they look at it.
Aug
16
comment Inaccessible cardinals and Andrew Wiles's proof
Actually, Cosmonut misquoted the article by leaving out the rest of the statement: "But there is a general consensus among mathematicians that this was just a convenient short cut rather than a logical necessity. With a little work, Wiles's proof should be translatable into Peano arithmetic or some slight extension of it."
Aug
6
awarded  Enlightened
Aug
6
awarded  Nice Answer
Aug
4
awarded  Good Answer
Jul
29
comment How connected are you?
MR collaboration numbers aren't accurate. For example, MR claims my collaboration number with Gauss is an improbable 6. They get that number by claiming Landau and Riemann are coauthors and Riemann and Gauss are coauthors because works by them were included together in reprint collections. Similarly, they incorrectly give my Erdos number as 3 because they count me as a co-author of Gerardin when we only published articles in the same collection.
Jul
9
awarded  Enlightened
Jul
7
answered Jordan decomposition in a classical group
Jul
7
awarded  Nice Answer
Jul
7
comment Does the Tannaka-Krein theorem come from an equivalence of 2-categories?
@Theo et al. --- well, you could try looking up the definition in Catégories Tannakiennes (Saavedra 1972, Deligne 1990) or Tannakian Categories (Deligne and ... 1982, Breen 1994) or .. A Tannakian category over a field $k$ is neutral if it admits a fibre functor over $k$. In general, it only admits a fibre functor over an extension of $k$. There are various expressions of Tannaka duality in 2-category terms in Saavedra, e.g., III 2.3.2, p180.
Jun
25
comment Weight filtration over the integers
algori. Thanks! you have helped clarify things for me.
Jun
25
comment Weight filtration over the integers
Actually, I'm still confused by Voisin, since in that section she seems to be talking about complex manifolds, not algebraic varieties. Perhaps the assumption is hidden somewhere.