1,684 research outputs found
Generic design of Chinese remaindering schemes
We propose a generic design for Chinese remainder algorithms. A Chinese
remainder computation consists in reconstructing an integer value from its
residues modulo non coprime integers. We also propose an efficient linear data
structure, a radix ladder, for the intermediate storage and computations. Our
design is structured into three main modules: a black box residue computation
in charge of computing each residue; a Chinese remaindering controller in
charge of launching the computation and of the termination decision; an integer
builder in charge of the reconstruction computation. We then show that this
design enables many different forms of Chinese remaindering (e.g.
deterministic, early terminated, distributed, etc.), easy comparisons between
these forms and e.g. user-transparent parallelism at different parallel grains
Adaptive Triangular System Solving
Large-scale applications and software systems are
getting increasingly complex. To deal with this complexity, those
systems must manage themselves in accordance with high-level guidance
from humans. Adaptive and hybrid algorithms enable this
self-management of resources and structured inputs.
In this talk, we first propose a classification of the different
notions of adaptivity. For us, an algorithm is adaptive (or a
poly-algorithm) when there is a choice at a high level between at
least two distinct algorithms, each of which could solve the same
problem. The choice is strategic, not tactical. It is motivated by
an increase of the performance of the execution, depending on both
input/output data and computing resources.
Then we propose a new adaptive algorithm for the exact simultaneous
resolution of several triangular systems over finite fields. The
resolution of such systems is e.g. one of the two main operations in block
Gaussian elimination. For solving triangular systems over finite
fields, the block algorithm reduces to matrix multiplication and
achieves the best known algebraic complexity. Exact matrix
multiplication, together with matrix factorizations, over finite
fields can now be performed at the speed of the highly optimized
numerical BLAS routines. This has been established by the FFLAS and
FFPACK libraries. In this talk we propose several practicable variants
solving these systems: a pure recursive version, a reduction to the
numerical dtrsm routine and a delaying of the modulus operation. Then
a cascading scheme is proposed to merge these variants into an
adaptive sequential algorithm.
We then propose a parallelization of this resolution. The adaptive
sequential algorithm is not the best parallel algorithm since its
recursion induces a dependancy. A better parallel algorithm would be
to first invert the matrix and then to multiply this inverse by the
right hand side. Unfortunately the latter requires more total
operations than the adaptive algorithm. We thus propose a coupling of
the sequential algorithm and of the parallel one in order to get the
best performances on any number of processors. The resulting cascading
is then an adaptation to resources.
This shows that the same process has been used both for adaptation to
data and to resources. We thus propose a generic framework for the
automatic adaptation of algorithms using recursive cascading
Spin-Polarized Electrons in Monolayer MoS
The optical susceptibility is a local, minimally-invasive and spin-selective
probe of the ground state of a two-dimensional electron gas. We apply this
probe to a gated monolayer of MoS. We demonstrate that the electrons are
spin polarized. Of the four available bands, only two are occupied. These two
bands have the same spin but different valley quantum numbers. We argue that
strong Coulomb interactions are a key aspect of this spontaneous symmetry
breaking. The Bohr radius is so small that even electrons located far apart in
phase space interact, facilitating exchange couplings to align the spins
Quantum confined Stark effect in a MoS monolayer van der Waals heterostructure
The optics of dangling-bond-free van der Waals heterostructures containing
transition metal dichalcogenides are dominated by excitons. A crucial property
of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a
heterostructure is used to probe the QCSE by applying a uniform vertical
electric field across a molybdenum disulfide (MoS) monolayer. The
photoluminescence emission energies of the neutral and charged excitons shift
quadratically with the applied electric field provided the electron density
remains constant, demonstrating that the exciton can be polarized. Stark shifts
corresponding to about half the homogeneous linewidth were achieved. Neutral
and charged exciton polarizabilities of (7.8~\pm~1.0)\times
10^{-10}~\tr{D~m~V}^{-1} and (6.4~\pm~0.9)\times 10^{-10}~\tr{D~m~V}^{-1} at
relatively low electron density (8 \times 10^{11}~\tr{cm}^{-2}) have been
extracted, respectively. These values are one order of magnitude lower than the
previously reported values, but in line with theoretical calculations. The
methodology presented here is versatile and can be applied to other
semiconducting layered materials as well
First-order magnetic phase-transition of mobile electrons in monolayer MoS
Evidence is presented for a first-order magnetic phase transition in a gated
two-dimensional semiconductor, monolayer-MoS. The phase boundary separates
a spin-polarised (ferromagnetic) phase at low electron density and a
paramagnetic phase at high electron density. Abrupt changes in the optical
response signal an abrupt change in the magnetism. The magnetic order is
thereby controlled via the voltage applied to the gate electrode of the device.
Accompanying the change in magnetism is a large change in the electron
effective mass
Recursion based parallelization of exact dense linear algebra routines for Gaussian elimination
International audienceWe present block algorithms and their implementation for the parallelization of sub-cubic Gaussian elimination on shared memory architectures.Contrarily to the classical cubic algorithms in parallel numerical linear algebra, we focus here on recursive algorithms and coarse grain parallelization.Indeed, sub-cubic matrix arithmetic can only be achieved through recursive algorithms making coarse grain block algorithms perform more efficiently than fine grain ones. This work is motivated by the design and implementation of dense linear algebraover a finite field, where fast matrix multiplication is used extensively and where costly modular reductions also advocate for coarse grain block decomposition. We incrementally build efficient kernels, for matrix multiplication first, then triangular system solving, on top of which a recursive PLUQ decomposition algorithm is built. We study the parallelization of these kernels using several algorithmic variants: either iterative or recursive and using different splitting strategies. Experiments show that recursive adaptive methods for matrix multiplication, hybrid recursive-iterative methods for triangular system solve and tile recursive versions of the PLUQ decomposition, together with various data mapping policies, provide the best performance on a 32 cores NUMA architecture. Overall, we show that the overhead of modular reductions is more than compensated by the fast linear algebra algorithms and that exact dense linear algebra matches the performance of full rank reference numerical software even in the presence of rank deficiencies
Impact of Tropical Climate on Selective Attention and Affect
Heat has an impact on several aspects of human cognition but the effects of the tropical climate (i.e., hot and wet) have rarely been explored. The purpose of this study was to determine whether selective attention and affect are negatively impacted by the tropical climate. The study followed a within-participants design: participants responded to an affective scale (PANAS) and performed an attention task (d2 Test) in two experimental climate conditions (tropical vs. neutral) with a one-week interval between sessions. The results indicated that they had lower positive affect and selective attention in the tropical climate than in the neutral climate. However, there was no significant difference in the effect on negative affect between conditions. The impact of tropical climate on affects and selective attention is discussed
Optical second harmonic generation in encapsulated single-layer InSe
We report the observation of optical second harmonic generation (SHG) in
single-layer indium selenide (InSe). We measure a second harmonic signal of
under nonresonant excitation using a home-built
confocal microscope and a standard pulsed pico-second laser. We demonstrate
that polarization-resolved SHG serves as a fast, non-invasive tool to determine
the crystal axes in single-layer InSe and to relate the sharp edges of the
flake to the armchair and zigzag edges of the crystal structure. Our experiment
determines these angles to an accuracy better than .
Treating the two-dimensional material as a nonlinear polarizable sheet, we
determine a second-order nonlinear sheet polarizability for single-layer InSe, corresponding to an effective nonlinear
susceptibility value of accounting for the sheet
thickness ( ). We demonstrate that the SHG
technique can also be applied to encapsulated samples to probe their crystal
orientations. The method is therefore suitable for creating high quality van
der Waals heterostructures with control over the crystal directions
SPODT: An R Package to Perform Spatial Partitioning
International audienceSpatial cluster detection is a classical question in epidemiology: Are cases located near other cases? In order to classify a study area into zones of different risks and determine their boundaries, we have developed a spatial partitioning method based on oblique decision trees, which is called spatial oblique decision tree (SpODT). This non-parametric method is based on the classification and regression tree (CART) approach introduced by Leo Breiman. Applied to epidemiological spatial data, the algorithm recursively searches among the coordinates for a threshold or a boundary between zones, so that the risks estimated in these zones are as different as possible. While the CART algorithm leads to rectangular zones, providing perpendicular splits of longitudes and latitudes, the SpODT algorithm provides oblique splitting of the study area, which is more appropriate and accurate for spatial epidemiology. Oblique decision trees can be considered as non-parametric regression models. Beyond the basic function, we have developed a set of functions that enable extended analyses of spatial data, providing: inference, graphical representations, spatio-temporal analysis, adjustments on covariates, spatial weighted partition, and the gathering of similar adjacent final classes. In this paper, we propose a new R package, SPODT, which provides an extensible set of functions for partitioning spatial and spatio-temporal data. The implementation and extensions of the algorithm are described. Function usage examples are proposed, looking for clustering malaria episodes in Bandiagara, Mali, and samples showing three different cluster shapes
An Improved Itinerary Recording Protocol for Securing Distributed Architectures Based on Mobile Agents
This paper proposes an improved itinerary recording protocol for securing distributed architectures based on mobile agents. The behavior of each of the cooperating agents is described, as well as the decision process establishing the identities of offenders when an attack is detected. Our protocol is tested on a set of potential attacks and the results confirm our assumption regarding offender designations and moments of detection. More precisely, the performance evaluation shows that our protocol detects the attack where there is collaboration between a platform on the cooperating agents' itinerary and another on the mobile agent's itinerary. As a result, this protocol constitutes a suitable option for electronic commerce applications where security concerns prevail over cost factors
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