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17
votes
1
answer
7k
views
A nice explanation of what is a smooth (l-adic) sheaf?
I would like to understand this concept. It seems to be important (for the theory of perverse sheaves), yet I don't know any nice exposition of the properties of smooth sheaves.
15
votes
1
answer
2k
views
What is the purpose of section 3 of BBD?
I am not quite sure that this question is appropriate for Mathoverflow, yet I would be deeply grateful for any hint: what happens in section 3 of Beilinson A., Bernstein J., Deligne P., Faisceaux perv …
10
votes
1
answer
1k
views
Bad behaviour of perverse sheaves over 'general' bases?
Could one define $\mathbb{Q}_l$-perverse etale sheaves over more or less general (excellent, separated) base scheme by combining the results of Gabber and Ekedahl? Would their functoriality properties …
9
votes
3
answers
2k
views
Applications for intersection (co)homology and for the Decomposition Theorem for students?
Which applications of intersection (co)homology and of the (Topological) Decomposition Theorem have most chances to be understood by students?
9
votes
0
answers
332
views
Is it possible to define a perverse $t$-structure for a certain triangulated category of she...
The perverse t-structure for the derived category of complexes of sheaves is certainly a mighty tool for studying cohomology. My question is: does there exist any homotopy-theoretic analogue for it (p …
8
votes
1
answer
722
views
The conjectural relation between mixed motivic sheaves and the perverse t-structure.
As far as I remember, there 'should exist' an exact etale realization functor from the category of mixed motivic sheaves (over a base scheme $S$) to the category of perverse $l$-adic sheaves over $S$. …
7
votes
2
answers
699
views
Which statements in section 5 of BBD will fail if we consider $\mathbb{Q}_l$-adic sheaves th...
A stupid question: which statements in section 5 of BBD will fail if we replace $\overline{\mathbb{Q}_l}$-sheaves by just $\mathbb{Q}_l$-ones? I am especially interested in Proposition 5.1.15.
BBD = …
7
votes
2
answers
2k
views
In what setting does one usually define mixed sheaves and weights for them?
In BBD mixed sheaves and weights for them were only defined for ($\overline{\mathbb{Q}_l}$-)sheaves over a variety $X_0$ defined over a finite field $F$. Weights start to behave better when one extend …
5
votes
0
answers
734
views
Do all the main properties of constructible and perverse sheaves (in an 'arithmetic' situati...
This question is a continuation of Bad behaviour of perverse sheaves over 'general' bases?
Let $S$ (for example) be a finite type separated scheme over $\mathbb{Z}$. I would like: (1) to define the p …
5
votes
1
answer
528
views
Functoriality properties of the perverse $t$-structure for torsion (constructible complexes ...
I would like to apply the usual 'functoriality properties' of the perverse $t$-structure to torsion (constructible complexes of) sheaves (I am in the algebraic setting, so these are etale sheaves, bu …
4
votes
1
answer
632
views
Morphisms between pure complexes of sheaves
I would like to understand the theory of pure complexes of (etale?) sheaves (of geometric origin?). In particular, I would like to understand which conditions are realy necessary in (part 1 of) Theore …
3
votes
0
answers
232
views
What sorts of weights for perverse sheaves were or can be computed?
I am studying certain weights for (triangulated categories of relative) motives. Those are interesting; yet one can hardly say that they are very much explicit or effectively computable. So, I would l …
3
votes
1
answer
753
views
Is there a 'classical' definition for the support of a perverse sheaves.
I would like to define the support of a mixed motivic sheaf. This should be something similar to the support of a perverse sheaf.:) Is there any 'classical' definition for the latter?
I suspect that …
2
votes
DG enhancements of $\ell$-adic derived categories
Q1. So, I suggest you the following plan of the proof.
Note that any Verdier localization of a triangulated category possessing a differential graded enhancement possesses a differential graded enha …
2
votes
1
answer
522
views
Is there an easy proof of the fact that the intermediate image functor respects weights?
It was proven in BBD (see Corollary 5.3.2) that for an open immersion $j$ the functor $j_{!*}$ preserves weights of mixed sheaves. The proof relies on several previous results; it is especially compli …