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seen Jan 25 at 2:52

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
Dec
14
comment Does the $(\mathbb Z/2)$-graded isomorphism $E_n \cong E_{n+2}$ have any nice properties?
I didn't mean anything too metaphysical. Since the two-periodic E_n and E_{n+2} operads have actions of O(n) and O(n+2), something you could add to your list of hypothetical "nice properties" is that the isomorphism between them is equivariant for the inclusion O(n) --> O(n+2). Is that true?
Dec
14
comment Does the $(\mathbb Z/2)$-graded isomorphism $E_n \cong E_{n+2}$ have any nice properties?
Is there a single operad called E_n? Or is there an interesting connected space of operads called E_n with no distinguished base point? For the nonlinear version (operads in spaces), I hope that the "classifying space of E_n operads" is BO(n). The classifying space of E_n operads in your funny 2-periodic category must be a lot different. Maybe it is BO?
Nov
24
comment Is there a cotangent bundle of a stable $\infty$-category?
Thanks Sam. What if I insist that the construction of $T^* C$ should not depend on any monoidal structure $\otimes$? For example it doesn't seem like such a structure is available in part 2.
Nov
21
awarded  Nice Question
Nov
20
asked Is there a cotangent bundle of a stable $\infty$-category?
Nov
8
accepted When does a cubic surface pass through five lines?
Nov
7
awarded  Popular Question
Nov
6
asked When does a cubic surface pass through five lines?
Nov
6
comment RO(G) grading of Mackey functors
In characteristic zero the category of Mackey functors is semisimple. The simple Mackey functors over any field have been classified by Thevenaz and Webb -- they are one-to-one with conjugacy classes of pairs (subgroup $H$, irrep $L$ of $N_G(H)/H$). I think a sphere $\,\, S^V$ acts on the simple Mackey functor corresponding to $(H,L)$ by a shift, but a shift that depends on $H$: the dimension of the fixed-point space $V^H$.
Nov
1
awarded  Nice Question
Oct
23
awarded  Yearling
Jul
2
awarded  Curious
Oct
23
awarded  Yearling