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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.
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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?
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 $S$. …
Mikhail Bondarko's user avatar
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 = …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar
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 …
Mikhail Bondarko's user avatar

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