22,104 research outputs found
Local diophantine properties of modular curves of -elliptic sheaves
We study the existence of rational points on modular curves of
-elliptic sheaves over local fields and the structure of special
fibres of these curves. We discuss some applications which include finding
presentations for arithmetic groups arising from quaternion algebras, finding
the equations of modular curves of -elliptic sheaves, and constructing
curves violating the Hasse principle.Comment: 24 page
Perverse coherent sheaves and the geometry of special pieces in the unipotent variety
Let X be a scheme of finite type over a Noetherian base scheme S admitting a
dualizing complex, and let U be an open subset whose complement has codimension
at least 2. We extend the Deligne-Bezrukavnikov theory of perverse coherent
sheaves by showing that a coherent middle extension (or intersection
cohomology) functor from perverse sheaves on U to perverse sheaves on X may be
defined for a much broader class of perversities than has previously been
known. We also introduce a derived category version of the coherent middle
extension functor.
Under suitable hypotheses, we introduce a construction (called
"S2-extension") in terms of perverse coherent sheaves of algebras on X that
takes a finite morphism to U and extends it in a canonical way to a finite
morphism to X. In particular, this construction gives a canonical
"S2-ification" of appropriate X. The construction also has applications to the
"Macaulayfication" problem, and it is particularly well-behaved when X is
Gorenstein.
Our main goal, however, is to address a conjecture of Lusztig on the geometry
of special pieces (certain subvarieties of the unipotent variety of a reductive
algebraic group). The conjecture asserts in part that each special piece is the
quotient of some variety (previously unknown in the exceptional groups and in
positive characteristic) by the action of a certain finite group. We use
S2-extension to give a uniform construction of the desired variety.Comment: 30 pages; minor corrections and addition
Local -monodromy and level fixing
We tackle three related problems. The first deals with freeness of localized cohomology groups of Harris-Taylor perverse sheaves, defined on the special fiber of some Kottwitz-Harris-Taylor Shimura variety. We then study the nilpotent monodromy operator acting both on the global cohomology of KHT Shimura variety and on the perverse sheaf of vanishing cycles. We then exhibe cases of level fixing phenomenon in the sense where the level at some fixed place of any rise in characteristic zero of an irreducible automorphic representation, is fixed equals to the one modulo l
On generalised Deligne--Lusztig constructions
This thesis is on the representations of connected reductive groups over finite quotients of a complete discrete valuation ring. Several aspects of higher Deligne–Lusztig representations are studied.
First we discuss some properties analogous to the finite field case; for example, we show that the higher Deligne–Lusztig inductions are compatible with the Harish-Chandra inductions.
We then introduce certain subvarieties of higher Deligne–Lusztig varieties, by taking pre-images of lower level groups along reduction maps; their constructions are motivated by efforts on computing the representation dimensions. In special cases we show that their cohomologies are closely related to the higher Deligne–Lusztig representations.
Then we turn to our main results. We show that, at even levels the higher Deligne–Lusztig representations of general linear groups coincide with certain explicitly induced representations; thus in this case we solved a problem raised by Lusztig. The generalisation of this result for a general reductive group is completed jointly with Stasinski; we also present this generalisation. Some discussions on the relations between this result and the invariant characters of finite Lie algebras are also presented.
In the even level case, we give a construction of generic character sheaves on reductive groups over rings, which are certain complexes whose associated functions are higher Deligne–Lusztig characters; they are accompanied with induction and restriction functors. By assuming some properties concerning perverse sheaves, we show that the induction and restriction functors are transitive and admit a Frobenius reciprocity
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