4,259 research outputs found
Torsion functors with monomial support
The dependence of torsion functors on their supporting ideals is
investigated, especially in the case of monomial ideals of certain subrings of
polynomial algebras over not necessarily Noetherian rings. As an application it
is shown how flatness of quasicoherent sheaves on toric schemes is related to
graded local cohomology.Comment: updated reference
Graded and Filtered Fiber Functors on Tannakian Categories
We study fiber functors on Tannakian categories which are equipped with a
grading or a filtration. Our goal is to give a comprehensive set of
foundational results about such functors. A main result is that each filtration
on a fiber functor can be split by a grading fpqc-locally on the base scheme
Decomposing locally compact groups into simple pieces
We present a contribution to the structure theory of locally compact groups.
The emphasis is on compactly generated locally compact groups which admit no
infinite discrete quotient. It is shown that such a group possesses a
characteristic cocompact subgroup which is either connected or admits a
non-compact non-discrete topologically simple quotient. We also provide a
description of characteristically simple groups and of groups all of whose
proper quotients are compact. We show that Noetherian locally compact groups
without infinite discrete quotient admit a subnormal series with all
subquotients compact, compactly generated Abelian, or compactly generated
topologically simple. Two appendices introduce results and examples around the
concept of quasi-product.Comment: Index added; minor change
Analytic vectors in continuous p-adic representations
Given a compact p-adic Lie group G over a finite unramified extension L/Q_p
let G_0 be the product over all Galois conjugates of G. We construct an exact
and faithful functor from admissible G-Banach space representations to
admissible locally L-analytic G_0-representations that coincides with passage
to analytic vectors in case L=Q_p. On the other hand, we study the functor
"passage to analytic vectors" and its derived functors over general basefields.
As an application we determine the higher analytic vectors in certain locally
analytic induced representations.Comment: Final version (appeared in Comp. Math. 2009). Exposition shortened.
Minor items correcte
Asymptotical behaviour of roots of infinite Coxeter groups
Let W be an infinite Coxeter group. We initiate the study of the set E of
limit points of "normalized" positive roots (representing the directions of the
roots) of W. We show that E is contained in the isotropic cone of the bilinear
form B associated to a geometric representation, and illustrate this property
with numerous examples and pictures in rank 3 and 4. We also define a natural
geometric action of W on E, and then we exhibit a countable subset of E, formed
by limit points for the dihedral reflection subgroups of W. We explain that
this subset is built from the intersection with Q of the lines passing through
two positive roots, and finally we establish that it is dense in E.Comment: 19 pages, 11 figures. Version 2: 29 pages, 11 figures. Reorganisation
of the paper, addition of many details (section 5 in particular). Version 3 :
revised edition accepted in Journal of the CMS. The number "I" was removed
from the title since number "II" paper was named differently, see
http://arxiv.org/abs/1303.671
Invariant functions on symplectic representations
Let G be a connected reductive group. In this paper we are studying the
invariant theory of symplectic G-modules. Our main result is that the invariant
moment map is equidimensional. We deduce that the categorical quotient is a
fibration over an affine space with rational generic fibers. Of particular
interest are those modules for which the generic orbit is coisotropic. We prove
that they are cofree. This result has been used in another paper
(math.SG/0505268) to classify all these modules. Our main tool is a symplectic
version of the local structure theorem.Comment: v1: 24 pages; v2: 31 pages, expanded exposition, new introduction,
some facts (esp. Thm. 7.2+Corollaries, Thm. 8.4) which were only implicit in
v1 are now spelled ou
On the Chabauty space of locally compact abelian groups
This paper contains several results about the Chabauty space of a general
locally compact abelian group. Notably, we determine its topological dimension,
we characterize when it is totally disconnected or connected; we characterize
isolated points.Comment: 24 pages, 0 figur
Coxeter groups, imaginary cones and dominance
Brink and Howlett have introduced a partial ordering, called dominance, on
the positive roots in the Tits realization of Coxeter groups (Math. Ann. 296
(1993), 179--190). Recently a concept called -height is introduced to
each reflection in an arbitrary Coxeter group (Edgar, Dominance and
regularity in Coxeter groups, PhD thesis, 2009). It is known (Dyer,
unpublished) that for all of finite rank, and for each non-negative ,
the set of reflections of -height equal to is finite. However, it
is not clear that the concepts of -height and dominance are related.
Here we show that the -height of an arbitrary reflection is equal to
the number of positive roots strictly dominated by the positive root
corresponding to that reflection. We also give applications of dominance to the
study of imaginary cones of Coxeter groups
The -homology of even Coxeter groups
Given a Coxeter system (W,S), there is an associated CW-complex, Sigma, on
which W acts properly and cocompactly. We prove that when the nerve L of (W,S)
is a flag triangulation of the 3-sphere, then the reduced -homology of
Sigma vanishes in all but the middle dimension.Comment: 15 pages, 1 figur
Twist-rigid Coxeter groups
We prove that two angle-compatible Coxeter generating sets of a given
finitely generated Coxeter group are conjugate provided one of them does not
admit any elementary twist. This confirms a basic case of a general conjecture
which describes a potential solution to the isomorphism problem for Coxeter
groups.Comment: 28 pages, 1 figur
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