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seen Jul 27 at 6:24

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?
Mar
16
comment What if the low-degree cohomology of a $G$-module and all its restrictions vanish?
It looks like this is a 1959 result of Rim. Thanks Chris.
Mar
16
accepted What if the low-degree cohomology of a $G$-module and all its restrictions vanish?
Mar
15
asked What if the low-degree cohomology of a $G$-module and all its restrictions vanish?
Feb
10
awarded  Popular Question
Feb
9
answered Calculating Mayer-Vietoris efficiently
Feb
9
comment Calculating Mayer-Vietoris efficiently
"Suppose that I know all of the $H^*(U_i)$'s, and all of the restriction maps between them, and I would like to compute $H^*(X)$." Note that even when there are just two open sets $U_1$ and $U_2$ and their intersection $U_1 \cap U_2$, this data only determines one-third of the maps in the Mayer-Vietoris long exact sequence. You get an upper bound on $H^*(U_1 \cup U_2)$ but does not determine it in general. A similar problem with the Mayer-Vietoris spectral sequence, is that the data you have supplied is enough to determine the groups on the $E_2$ page but none of the differentials.
Jan
23
accepted Is mean width a Dehn invariant?
Jan
23
comment Is mean width a Dehn invariant?
Ilya, what is the mean width of the 1x1xN^3 box?
Jan
23
comment Is mean width a Dehn invariant?
Hi Ilya and Yoav. Have I misunderstood the definition of scissors congruence, or the discussion of mean width on this wikipedia page? en.wikipedia.org/wiki/Hadwiger_theorem
Jan
23
asked Is mean width a Dehn invariant?
Jan
13
comment Exact Functors from Perverse Sheaves
If $0 \to P' \to P \to P'' \to 0$ is a short exact sequence of perverse sheaves, then there is a map $P'' \to P'[1]$ in the derived category making $P' \to P \to P'' \to P'[1]$ into an exact triangle. (I think this is the definition of "admissible" abelian subcategories of triangulated categories given by Beilinson-Bernstein-Deligne section 1.2; in section 1.3 they show the heart of a t-structure is admissible).
Jan
12
comment Twisting of the power functor
Hi Dmitry. What does the symbol "D^b(T(k))" mean?
Dec
17
comment automorphism of prime order for group of Lie type in
There's an automorphism of Spin(8), of order 3, that does not preserve any pinning. Its fixed-point subgroup is SL(3). I don't know whether it exists in characteristic 3. There is a nice treatment of all characteristic zero examples at www2.bc.edu/mark-reeder/Torsion.pdf