855 research outputs found
Geometric representation of interval exchange maps over algebraic number fields
We consider the restriction of interval exchange transformations to algebraic
number fields, which leads to maps on lattices. We characterize
renormalizability arithmetically, and study its relationships with a
geometrical quantity that we call the drift vector. We exhibit some examples of
renormalizable interval exchange maps with zero and non-zero drift vector, and
carry out some investigations of their properties. In particular, we look for
evidence of the finite decomposition property: each lattice is the union of
finitely many orbits.Comment: 34 pages, 8 postscript figure
Symbolic approach and induction in the Heisenberg group
We associate a homomorphism in the Heisenberg group to each hyperbolic
unimodular automorphism of the free group on two generators. We show that the
first return-time of some flows in "good" sections, are conjugate to
niltranslations, which have the property of being self-induced.Comment: 18 page
Impacts of changes in groundwater recharge on the isotopic composition and geochemistry of seasonally ice-covered lakes: insights for sustainable management
Lakes are under increasing pressure due to widespread anthropogenic impacts
related to rapid development and population growth. Accordingly, many lakes
are currently undergoing a systematic decline in water quality. Recent
studies have highlighted that global warming and the subsequent changes in
water use may further exacerbate eutrophication in lakes. Lake evolution
depends strongly on hydrologic balance, and therefore on groundwater
connectivity. Groundwater also influences the sensitivity of lacustrine
ecosystems to climate and environmental changes, and governs their
resilience. Improved characterization of groundwater exchange with lakes is
needed today for lake preservation, lake restoration, and sustainable
management of lake water quality into the future. In this context, the aim of
the present paper is to determine if the future evolution of the climate, the
population, and the recharge could modify the geochemistry of lakes (mainly
isotopic signature and quality via phosphorous load) and if the isotopic
monitoring of lakes could be an efficient tool to highlight the variability
of the water budget and quality.
Small groundwater-connected lakes were chosen to simulate changes in water
balance and water quality expected under future climate change scenarios,
namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary
baseline conditions, including isotope mass balance and geochemical
characteristics, were determined through an intensive field-based research
program prior to the simulations. Results highlight that future lake
geochemistry and isotopic composition trends will depend on four main
parameters: location (and therefore climate conditions), lake catchment size
(which impacts the intensity of the flux change), lake volume (which impacts
the range of variation), and lake G index (i.e., the percentage of
groundwater that makes up total lake inflows), the latter being the dominant
control on water balance conditions, as revealed by the sensitivity of lake
isotopic composition. Based on these model simulations, stable isotopes
appear to be especially useful for detecting changes in recharge to lakes
with a G index of between 50 and 80 %, but response is non-linear.
Simulated monthly trends reveal that evolution of annual lake isotopic
composition can be dampened by opposing monthly recharge fluctuations. It is
also shown that changes in water quality in groundwater-connected lakes
depend significantly on lake location and on the intensity of recharge
change
Critical connectedness of thin arithmetical discrete planes
An arithmetical discrete plane is said to have critical connecting thickness
if its thickness is equal to the infimum of the set of values that preserve its
-connectedness. This infimum thickness can be computed thanks to the fully
subtractive algorithm. This multidimensional continued fraction algorithm
consists, in its linear form, in subtracting the smallest entry to the other
ones. We provide a characterization of the discrete planes with critical
thickness that have zero intercept and that are -connected. Our tools rely
on the notion of dual substitution which is a geometric version of the usual
notion of substitution acting on words. We associate with the fully subtractive
algorithm a set of substitutions whose incidence matrix is provided by the
matrices of the algorithm, and prove that their geometric counterparts generate
arithmetic discrete planes.Comment: 18 pages, v2 includes several corrections and is a long version of
the DGCI extended abstrac
Cross sections for geodesic flows and \alpha-continued fractions
We adjust Arnoux's coding, in terms of regular continued fractions, of the
geodesic flow on the modular surface to give a cross section on which the
return map is a double cover of the natural extension for the \alpha-continued
fractions, for each in (0,1]. The argument is sufficiently robust to
apply to the Rosen continued fractions and their recently introduced
\alpha-variants.Comment: 20 pages, 2 figure
Impacts of changes in groundwater recharge on the isotopic composition and geochemistry of seasonally ice-covered lakes: insights for sustainable management
Lakes are under increasing pressure due to widespread anthropogenic impacts
related to rapid development and population growth. Accordingly, many lakes
are currently undergoing a systematic decline in water quality. Recent
studies have highlighted that global warming and the subsequent changes in
water use may further exacerbate eutrophication in lakes. Lake evolution
depends strongly on hydrologic balance, and therefore on groundwater
connectivity. Groundwater also influences the sensitivity of lacustrine
ecosystems to climate and environmental changes, and governs their
resilience. Improved characterization of groundwater exchange with lakes is
needed today for lake preservation, lake restoration, and sustainable
management of lake water quality into the future. In this context, the aim of
the present paper is to determine if the future evolution of the climate, the
population, and the recharge could modify the geochemistry of lakes (mainly
isotopic signature and quality via phosphorous load) and if the isotopic
monitoring of lakes could be an efficient tool to highlight the variability
of the water budget and quality.
Small groundwater-connected lakes were chosen to simulate changes in water
balance and water quality expected under future climate change scenarios,
namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary
baseline conditions, including isotope mass balance and geochemical
characteristics, were determined through an intensive field-based research
program prior to the simulations. Results highlight that future lake
geochemistry and isotopic composition trends will depend on four main
parameters: location (and therefore climate conditions), lake catchment size
(which impacts the intensity of the flux change), lake volume (which impacts
the range of variation), and lake G index (i.e., the percentage of
groundwater that makes up total lake inflows), the latter being the dominant
control on water balance conditions, as revealed by the sensitivity of lake
isotopic composition. Based on these model simulations, stable isotopes
appear to be especially useful for detecting changes in recharge to lakes
with a G index of between 50 and 80 %, but response is non-linear.
Simulated monthly trends reveal that evolution of annual lake isotopic
composition can be dampened by opposing monthly recharge fluctuations. It is
also shown that changes in water quality in groundwater-connected lakes
depend significantly on lake location and on the intensity of recharge
change
Next-generation seismic experiments – II: wide-angle, multi-azimuth, 3-D, full-waveform inversion of sparse field data
3-D full-waveform inversion (FWI) is an advanced seismic imaging technique that has been widely adopted by the oil and gas industry to obtain high-fidelity models of P-wave velocity that lead to improvements in migrated images of the reservoir. Most industrial applications of 3-D FWI model the acoustic wavefield, often account for the kinematic effect of anisotropy, and focus on matching the low-frequency component of the early arriving refractions that are most sensitive to P-wave velocity structure. Here, we have adopted the same approach in an application of 3-D acoustic, anisotropic FWI to an ocean-bottom-seismometer (OBS) field data set acquired across the Endeavour oceanic spreading centre in the northeastern Pacific. Starting models for P-wave velocity and anisotropy were obtained from traveltime tomography; during FWI, velocity is updated whereas anisotropy is kept fixed. We demonstrate that, for the Endeavour field data set, 3-D FWI is able to recover fine-scale velocity structure with a resolution that is 2–4 times better than conventional traveltime tomography. Quality assurance procedures have been employed to monitor each step of the workflow; these are time consuming but critical to the development of a successful inversion strategy. Finally, a suite of checkerboard tests has been performed which shows that the full potential resolution of FWI can be obtained if we acquire a 3-D survey with a slightly denser shot and receiver spacing than is usual for an academic experiment. We anticipate that this exciting development will encourage future seismic investigations of earth science targets that would benefit from the superior resolution offered by 3-D FWI
Escape orbits and Ergodicity in Infinite Step Billiards
In a previous paper we defined a class of non-compact polygonal billiards,
the infinite step billiards: to a given decreasing sequence of non-negative
numbers , there corresponds a table \Bi := \bigcup_{n\in\N} [n,n+1]
\times [0,p_{n}].
In this article, first we generalize the main result of the previous paper to
a wider class of examples. That is, a.s. there is a unique escape orbit which
belongs to the alpha and omega-limit of every other trajectory. Then, following
a recent work of Troubetzkoy, we prove that generically these systems are
ergodic for almost all initial velocities, and the entropy with respect to a
wide class of ergodic measures is zero.Comment: 27 pages, 8 figure
Optical Coatings as Mirrors for Optical Diagnostics
The aim of this work was to provide a comprehensive insight concerning coated films which might be used for first mirrors in ITER. The influence of the mirror crystallite size has been addressed as well as the coating techniques to provide nanocrystalline films. Tests of coated mirrors both in laboratories and in tokamaks are reviewed. For the tokamak tests a wide angle camera system has been installed in JET-ILW which is composed of a mirror box with 3 stainless steel mirrors coated with rhodium viewing the torus through a conically shaped aperture. The system delivered the required image quality for plasma monitoring and wall protection. No or insignificant degradation of the optical transmittance has been observed during the experimental campaign in 2014 with about 3000 plasma pulses in different magnetic field configurations
Triangulations and Severi varieties
We consider the problem of constructing triangulations of projective planes
over Hurwitz algebras with minimal numbers of vertices. We observe that the
numbers of faces of each dimension must be equal to the dimensions of certain
representations of the automorphism groups of the corresponding Severi
varieties. We construct a complex involving these representations, which should
be considered as a geometric version of the (putative) triangulations
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