7,376 research outputs found
Identifying cross country skiing techniques using power meters in ski poles
Power meters are becoming a widely used tool for measuring training and
racing effort in cycling, and are now spreading also to other sports. This
means that increasing volumes of data can be collected from athletes, with the
aim of helping coaches and athletes analyse and understanding training load,
racing efforts, technique etc. In this project, we have collaborated with
Skisens AB, a company producing handles for cross country ski poles equipped
with power meters. We have conducted a pilot study in the use of machine
learning techniques on data from Skisens poles to identify which "gear" a skier
is using (double poling or gears 2-4 in skating), based only on the sensor data
from the ski poles. The dataset for this pilot study contained labelled
time-series data from three individual skiers using four different gears
recorded in varied locations and varied terrain. We systematically evaluated a
number of machine learning techniques based on neural networks with best
results obtained by a LSTM network (accuracy of 95% correctly classified
strokes), when a subset of data from all three skiers was used for training. As
expected, accuracy dropped to 78% when the model was trained on data from only
two skiers and tested on the third. To achieve better generalisation to
individuals not appearing in the training set more data is required, which is
ongoing work.Comment: Presented at the Norwegian Artificial Intelligence Symposium 201
Zitterbewegung is not an observable
It has recently been claimed that Zitterbewegung has been observed. However,
we argue that it is not an observable and that the authors' observations must
be reinterpreted
Fermion Systems in Discrete Space-Time Exemplifying the Spontaneous Generation of a Causal Structure
As toy models for space-time on the Planck scale, we consider examples of
fermion systems in discrete space-time which are composed of one or two
particles defined on two up to nine space-time points. We study the
self-organization of the particles as described by a variational principle both
analytically and numerically. We find an effect of spontaneous symmetry
breaking which leads to the emergence of a discrete causal structure.Comment: 37 pages, LaTeX, 12 figures, minor changes (published version
Quantum derivation of the use of classical electromagnetic potentials in relativistic Coulomb excitation
We prove that a relativistic Coulomb excitation calculation in which the
classical electromagnetic field of the projectile is used to induce transitions
between target states gives the same target transition amplitudes, to all
orders of perturbation theory, as would a calculation in which the interaction
between projectile and target is mediated by a quantized electromagnetic field.Comment: 1 .zip file containing LaTex source plus three figures as .eps file
Star Formation and Dust Extinction Properties of Local Galaxies as seen from AKARI and GALEX
An accurate estimation of the star formation-related properties of galaxies
is crucial for understanding the evolution of galaxies. In galaxies,
ultraviolet (UV) light emitted by recently formed massive stars is attenuated
by dust, which is also produced by star formation (SF) activity, and is
reemitted at mid- and far- infrared (IR) wavelengths. In this study, we
investigate the star formation rate (SFR) and dust extinction using UV and IR
data. We selected local galaxies which are detected at AKARI FIS 90 um and
matched the IRAS IIFSCz 60 um select catalog. We measured FUV and NUV flux
densities from GALEX images. We examined the SF and extinction of Local
galaxies using four bands of AKARI. Then, we calculated FUV and total IR
luminosities, and obtained the SF luminosity, L_{SF}, the total luminosity
related to star formation activity, and the SFR. We find that in most galaxies,
L_{SF} is dominated by L_{dust}. We also find that galaxies with higher SF
activity have a higher fraction of their SF hidden by dust. In fact, the SF of
galaxies with SFRs >20 M_{sun}/yr is almost completely hidden by dust. Our
results boast a significantly higher precision with respect to previously
published works, due to the use of much larger object samples from the AKARI
and GALEX all sky surveys.Comment: 9 pages, 12 figures, accepted for publication in Earth, Planets, and
Space, A few minor corrections, and a reference adde
Theory of the thermoelectricity of intermetallic compounds with Ce or Yb ions
The thermoelectric properties of intermetallic compounds with Ce or Yb ions
are explained by the single-impurity Anderson model which takes into account
the crystal-field splitting of the 4{\it f} ground-state multiplet, and assumes
a strong Coulomb repulsion which restricts the number of {\it f} electrons or
{\it f} holes to for Ce and for Yb ions. Using
the non-crossing approximation and imposing the charge neutrality constraint on
the local scattering problem at each temperature and pressure, the excitation
spectrum and the transport coefficients of the model are obtained. The
thermopower calculated in such a way exhibits all the characteristic features
observed in Ce and Yb intermetallics. Calculating the effect of pressure on
various characteristic energy scales of the model, we obtain the phase
diagram which agrees with the experimental data on CeRuSi,
CeCuSi, CePdSi, and similar compounds. The evolution of the
thermopower and the electrical resistance as a function of temperature,
pressure or doping is explained in terms of the crossovers between various
fixed points of the model and the redistribution of the single-particle
spectral weight within the Fermi window.Comment: 13 pages, 11 figure
The microcanonical ensemble of the ideal relativistic quantum gas with angular momentum conservation
We derive the microcanonical partition function of the ideal relativistic
quantum gas with fixed intrinsic angular momentum as an expansion over fixed
multiplicities. We developed a group theoretical approach by generalizing known
projection techniques to the Poincare' group. Our calculation is carried out in
a quantum field framework and applies to particles with any spin. It extends
known results in literature in that it does not introduce any large volume
approximation and it takes particle spin fully into account. We provide
expressions of the microcanonical partition function at fixed multiplicities in
the limiting classical case of large volumes and large angular momenta and in
the grand-canonical ensemble. We also derive the microcanonical partition
function of the ideal relativistic quantum gas with fixed parity.Comment: 38 pages; minor corrections to the formulae for the published versio
Instability of Myelin Tubes under Dehydration: deswelling of layered cylindrical structures
We report experimental observations of an undulational instability of myelin
figures. Motivated by this, we examine theoretically the deformation and
possible instability of concentric, cylindrical, multi-lamellar membrane
structures. Under conditions of osmotic stress (swelling or dehydration), we
find a stable, deformed state in which the layer deformation is given by \delta
R ~ r^{\sqrt{B_A/(hB)}}, where B_A is the area compression modulus, B is the
inter-layer compression modulus, and h is the repeat distance of layers. Also,
above a finite threshold of dehydration (or osmotic stress), we find that the
system becomes unstable to undulations, first with a characteristic wavelength
of order \sqrt{xi d_0}, where xi is the standard smectic penetration depth and
d_0 is the thickness of dehydrated region.Comment: 5 pages + 3 figures [revtex 4
Scattering of Dirac electrons by circular mass barriers: valley filter and resonant scattering
The scattering of two-dimensional (2D) massless Dirac electrons is
investigated in the presence of a random array of circular mass barriers. The
inverse momentum relaxation time and the Hall factor are calculated and used to
obtain parallel and perpendicular resistivity components within linear
transport theory. We found a non zero perpendicular resistivity component which
has opposite sign for electrons in the different K and K' valleys. This
property can be used for valley filter purposes. The total cross-section for
scattering on penetrable barriers exhibit resonances due to the presence of
quasi-bound states in the barriers that show up as sharp gaps in the
cross-section while for Schr\"{o}dinger electrons they appear as peaks.Comment: 10 pages, 11 figure
Faithful fermionic representations of the Kondo lattice model
We study the Kondo lattice model using a class of canonical transformations
that allow us to faithfully represent the model entirely in terms of fermions
without constraints. The transformations generate interacting theories that we
study using mean field theory. Of particular interest is a new manifestly
O(3)-symmetric representation in terms of Majorana fermions at half-filling on
bipartite lattices. This representation suggests a natural O(3)-symmetric trial
state that is investigated and characterized as a gapped spin liquid.Comment: 11 pages, 2 figures, minor update
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