37,063 research outputs found
Near-tropical subsurface ice on Mars
Near-surface perennial water ice on Mars has been previously inferred down to
latitudes of about 45{\deg} and could result from either water vapor diffusion
through the regolith under current conditions or previous ice ages
precipitations. In this paper we show that at latitudes as low as 25{\deg} in
the southern hemisphere buried water ice in the shallow (< 1 m) subsurface is
required to explain the observed surface distribution of seasonal CO2 frost on
pole facing slopes. This result shows that possible remnants of the last ice
age, as well as water that will be needed for the future exploration of Mars,
are accessible significantly closer to the equator than previously thought,
where mild conditions for both robotic and human exploration lie
Feller property and infinitesimal generator of the exploration process
We consider the exploration process associated to the continuous random tree
(CRT) built using a Levy process with no negative jumps. This process has been
studied by Duquesne, Le Gall and Le Jan. This measure-valued Markov process is
a useful tool to study CRT as well as super-Brownian motion with general
branching mechanism. In this paper we prove this process is Feller, and we
compute its infinitesimal generator on exponential functionals and give the
corresponding martingale
Ultrafast spherulitic crystal growth as a stress-induced phenomenon specific of fragile glass-formers
We propose a model for the abrupt emergence, below temperatures close to the
glass transition, of the ultra-fast (GC) steady mode of spherulitic crystal
growth in deeply undercooled liquids. We interpret this phenomenon as
controlled by the interplay between the generation of stresses by
crystallization and their partial release by flow in the surrounding amorphous
visco-elastic matrix. Our model is consistent with both the observed ratios
() of fast-to-slow velocities and the fact that fast growth emerges
close to the glass transition. It leads us to conclude that the existence of a
fast growth regime requires both (i) a high fragility of the glassformer; (ii)
the fine sub-structure specific of spherulites. It finally predicts that the
transition is hysteretic, thus allowing for an independent experimental test
A constitutive model for unsaturated cemented soils under cyclic loading
On the basis of plastic bounding surface model, the damage theory for
structured soils and unsaturated soil mechanics, an elastoplastic model for
unsaturated loessic soils under cyclic loading has been elaborated. Firstly,
the description of bond degradation in a damage framework is given, linking the
damage of soil's structure to the accumulated strain. The Barcelona Basic Model
(BBM) was considered for the suction effects. The elastoplastic model is then
integrated into a bounding surface plasticity framework in order to model
strain accumulation along cyclic loading, even under small stress levels. The
validation of the proposed model is conducted by comparing its predictions with
the experimental results from multi-level cyclic triaxial tests performed on a
natural loess sampled beside the Northern French railway for high speed train
and about 140 km far from Paris. The comparisons show the capabilities of the
model to describe the behaviour of unsaturated cemented soils under cyclic
loading
Extension and calibration of a Hawkes-based optimal execution model
We provide some theoretical extensions and a calibration protocol for our
former dynamic optimal execution model. The Hawkes parameters and the
propagator are estimated independently on financial data from stocks of the
CAC40. Interestingly, the propagator exhibits a smoothly decaying form with one
or two dominant time scales, but only so after a few seconds that the market
needs to adjust after a large trade. Motivated by our estimation results, we
derive the optimal execution strategy for a multi-exponential Hawkes kernel and
backtest it on the data for round trips. We find that the strategy is
profitable on average when trading at the midprice, which is in accordance with
violated martingale conditions. However, in most cases, these profits vanish
when we take bid-ask costs into account
Error Analysis of Modified Langevin Dynamics
We consider Langevin dynamics associated with a modified kinetic energy
vanishing for small momenta. This allows us to freeze slow particles, and hence
avoid the re-computation of inter-particle forces, which leads to computational
gains. On the other hand, the statistical error may increase since there are a
priori more correlations in time. The aim of this work is first to prove the
ergodicity of the modified Langevin dynamics (which fails to be hypoelliptic),
and next to analyze how the asymptotic variance on ergodic averages depends on
the parameters of the modified kinetic energy. Numerical results illustrate the
approach, both for low-dimensional systems where we resort to a Galerkin
approximation of the generator, and for more realistic systems using Monte
Carlo simulations
Ecosystem Viable Yields
The World Summit on Sustainable Development (Johannesburg, 2002) encouraged
the application of the ecosystem approach by 2010. However, at the same Summit,
the signatory States undertook to restore and exploit their stocks at maximum
sustainable yield (MSY), a concept and practice without ecosystemic dimension,
since MSY is computed species by species, on the basis of a monospecific model.
Acknowledging this gap, we propose a definition of "ecosystem viable yields"
(EVY) as yields compatible i) with guaranteed biological safety levels for all
time and ii) with an ecosystem dynamics. To the difference of MSY, this notion
is not based on equilibrium, but on viability theory, which offers advantages
for robustness. For a generic class of multispecies models with harvesting, we
provide explicit expressions for the EVY. We apply our approach to the
anchovy--hake couple in the Peruvian upwelling ecosystem
The FC-rank of a context-free language
We prove that the finite condensation rank (FC-rank) of the lexicographic
ordering of a context-free language is strictly less than
A graph-based mathematical morphology reader
This survey paper aims at providing a "literary" anthology of mathematical
morphology on graphs. It describes in the English language many ideas stemming
from a large number of different papers, hence providing a unified view of an
active and diverse field of research
A generic tool to generate a lexicon for NLP from Lexicon-Grammar tables
Lexicon-Grammar tables constitute a large-coverage syntactic lexicon but they
cannot be directly used in Natural Language Processing (NLP) applications
because they sometimes rely on implicit information. In this paper, we
introduce LGExtract, a generic tool for generating a syntactic lexicon for NLP
from the Lexicon-Grammar tables. It is based on a global table that contains
undefined information and on a unique extraction script including all
operations to be performed for all tables. We also present an experiment that
has been conducted to generate a new lexicon of French verbs and predicative
nouns
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