205 research outputs found
On the Grothendieck-Serre Conjecture about principal bundles and its generalizations
Let be a regular connected affine semi-local scheme over a field . Let
be a reductive group scheme over . Assuming that has an appropriate
parabolic subgroup scheme, we prove the following statement. Given an affine
-scheme , a principal -bundle over is trivial if it is
trivial over the generic fiber of the projection . We also
simplify the proof of the Grothendieck-Serre conjecture: let be a regular
connected affine semi-local scheme over a field . Let be a reductive
group scheme over . A principal -bundle over is trivial if it is
trivial over the generic point of . We generalize some other related results
from the simple simply-connected case to the case of arbitrary reductive group
schemes.Comment: Final version to be published in the Journal of Algebra and Number
Theory. We slightly change the isotropy condition as well as the terminology:
see Definition 1.1 of strongly locally isotropic reductive groups. We remove
the assumption that U is geometrically regular in Theorem 1 (regular is
enough). Other minor improvement
On the Grothendieck-Serre conjecture on principal bundles in mixed characteristic
Let R be a regular local ring. Let G be a reductive R-group scheme. A
conjecture of Grothendieck and Serre predicts that a principal G-bundle over R
is trivial if it is trivial over the quotient field of R. The conjecture is
known when R contains a field. We prove the conjecture for a large class of
regular local rings not containing fields in the case when G is split.Comment: Minor corrections and improvement
Algebraic and hamiltonian approaches to isostokes deformations
We study a generalization of the isomonodromic deformation to the case of
connections with irregular singularities. We call this generalization Isostokes
Deformation. A new deformation parameter arises: one can deform the formal
normal forms of connections at irregular points. We study this part of the
deformation, giving an algebraic description. Then we show how to use loop
groups and hypercohomology to write explicit hamiltonians. We work on an
arbitrary complete algebraic curve, the structure group is an arbitrary
semisimiple group.Comment: 23 pages, minor corrections in the introduction, references expande
Generically isotropic reductive group schemes are locally isotropic
Let be a semilocal geometrically factorial Noetherian domain of
characteristic zero. We show that a reductive -group scheme is isotropic if
it is generically isotropic. We derive various consequences, in particular for
the Grothenieck-Serre conjecture and for homotopic invariance of torsors.Comment: Preliminary version. Comments are welcom
Estimating snow cover from publicly available images
In this paper we study the problem of estimating snow cover in mountainous
regions, that is, the spatial extent of the earth surface covered by snow. We
argue that publicly available visual content, in the form of user generated
photographs and image feeds from outdoor webcams, can both be leveraged as
additional measurement sources, complementing existing ground, satellite and
airborne sensor data. To this end, we describe two content acquisition and
processing pipelines that are tailored to such sources, addressing the specific
challenges posed by each of them, e.g., identifying the mountain peaks,
filtering out images taken in bad weather conditions, handling varying
illumination conditions. The final outcome is summarized in a snow cover index,
which indicates for a specific mountain and day of the year, the fraction of
visible area covered by snow, possibly at different elevations. We created a
manually labelled dataset to assess the accuracy of the image snow covered area
estimation, achieving 90.0% precision at 91.1% recall. In addition, we show
that seasonal trends related to air temperature are captured by the snow cover
index.Comment: submitted to IEEE Transactions on Multimedi
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