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Feb
9
accepted What's in the genus of the cubic lattice?
Feb
9
comment What's in the genus of the cubic lattice?
I think I didn't put it together until now, that odd unimodular lattices make a genus, and that's the genus containing $\mathbf{Z}^n$.
Feb
4
comment What's in the genus of the cubic lattice?
Thanks, Noam. -
Feb
4
comment What's in the genus of the cubic lattice?
Thanks for the O'Meara reference. The last footnote on the last page, referring to that Kneser work, is: "See M. Kneser (1957). For an example of the classical approach using 'reduction theory' we refer to BW Jones (1950)." I'd be interested to find out what O'M means, but I didn't find the Jones book yet.
Feb
3
comment What's in the genus of the cubic lattice?
Thanks Jeremy. How does magma do it? And is it the kind of thing that was known, or could have been, to Zolotareff?
Feb
3
asked What's in the genus of the cubic lattice?
Jan
31
revised Is there an approach to Gabber's theorem from the singular support of coherent sheaves?
"I heard" from somebody specific
Jan
30
awarded  Nice Question
Jan
30
revised Is there an approach to Gabber's theorem from the singular support of coherent sheaves?
I had "LE = E x BE" which is not quite right.
Jan
30
comment Is there an approach to Gabber's theorem from the singular support of coherent sheaves?
Hi Pavel. I think I am exactly asking why the equivariance implies Gabber's theorem. It doesn't seem like you get it from just any kind of circle action. Is it a special case of a theorem about coherent sheaves on a more general class of derived stacks?
Jan
28
asked Is there an approach to Gabber's theorem from the singular support of coherent sheaves?
Dec
19
comment elliptic curves and group cohomology
When $R = \mathbf{C}$, a line bundle on $M_G$ gives for each pair of commuting elements $(x,y)$ in $G$ a homomorphism $\rho:Z_G(x,y) \to \mathbf{C}^*$. If I represent $k$ as a $\mathbf{C}^*$-valued $3$-cocycle $k(g_1,g_2,g_3)$, is there an explicit formula for $\rho(g)$ in terms $k$? Perhaps $\rho(g) = k(x,y,g)$?
Oct
23
awarded  Yearling
Jul
23
comment highest weight the half-sum of positive roots
Is it the observation attributed to Kostant in the first two paragraphs here? mathoverflow.net/questions/14770/…
Jun
28
comment What is deforming this non-complete intersection like?
Thanks Will and Jason. Jason, can a non-CM ideal be flatly deformed to a CM ideal? Or are you telling me that this variety has no smooth deformations at all!
Jun
28
revised What is deforming this non-complete intersection like?
wrong equations
Jun
28
asked What is deforming this non-complete intersection like?
Mar
29
comment Has any attempt been made to classify finite groupoids?
Extensions of $G$ by a nonabelian $M$ are classified by cohomology of $G$ with coefficients in what the ancients called a crossed module; i.e. by homotopy classes of maps from $BG$ to the delooping of $\mathrm{Aut}(BM)$. Since $M_{12}$ has no center, $\mathrm{Aut}(BM_{12}) \cong \mathrm{Out}(BM_{12}) \cong \mathbf{Z}/2$. That means there can be no nontrivial extensions of a group like $\mathrm{SL}_3(\mathbf{F}_3)$ or $\mathrm{PGL}_3(\mathbf{F}_3)$ by $M_{12}$, right?
Mar
17
comment Does a polytope have a self-indexing shelling?
Thanks Richard, I hadn't seen this question. Besides not allowing deformations, another difference is that I am curious about shellings that do not come from linear functions.
Mar
17
asked Does a polytope have a self-indexing shelling?