28,398 research outputs found
Rule-based Machine Learning Methods for Functional Prediction
We describe a machine learning method for predicting the value of a
real-valued function, given the values of multiple input variables. The method
induces solutions from samples in the form of ordered disjunctive normal form
(DNF) decision rules. A central objective of the method and representation is
the induction of compact, easily interpretable solutions. This rule-based
decision model can be extended to search efficiently for similar cases prior to
approximating function values. Experimental results on real-world data
demonstrate that the new techniques are competitive with existing machine
learning and statistical methods and can sometimes yield superior regression
performance.Comment: See http://www.jair.org/ for any accompanying file
Quantum nature of cosmological bounces
Several examples are known where quantum gravity effects resolve the
classical big bang singularity by a bounce. The most detailed analysis has
probably occurred for loop quantum cosmology of isotropic models sourced by a
free, massless scalar. Once a bounce has been realized under fairly general
conditions, the central questions are how strongly quantum it behaves, what
influence quantum effects can have on its appearance, and what quantum
space-time beyond the bounce may look like. This, then, has to be taken into
account for effective equations which describe the evolution properly and can
be used for further phenomenological investigations. Here, we provide the first
analysis with interacting matter with new effective equations valid for weak
self-interactions or small masses. They differ from the free scalar equations
by crucial terms and have an important influence on the bounce and the
space-time around it. Especially the role of squeezed states, which have often
been overlooked in this context, is highlighted. The presence of a bounce is
proven for uncorrelated states, but as squeezing is a dynamical property and
may change in time, further work is required for a general conclusion.Comment: 26 page
Ground state energy of a homogeneous Bose-Einstein condensate beyond Bogoliubov
The standard calculations of the ground-state energy of a homogeneous Bose
gas rely on approximations which are physically reasonable but difficult to
control. Lieb and Yngvason [Phys. Rev. Lett. 80, 2504 (1998)] have proved
rigorously that the commonly accepted leading order term of the ground state
energy is correct in the zero-density-limit. Here, strong indications are given
that also the next to leading term is correct. It is shown that the first terms
obtained in a perturbative treatment provide contributions which are lost in
the Bogoliubov approach.Comment: 6 pages, accepted for publication in Europhys. Lett.
http://www.epletters.ch
Projections and Dyadic Parseval Frame MRA Wavelets
A classical theorem attributed to Naimark states that, given a Parseval frame
in a Hilbert space , one can embed in
a larger Hilbert space so that the image of is the
projection of an orthonormal basis for . In the present work, we
revisit the notion of Parseval frame MRA wavelets from two papers of
Paluszy\'nski, \v{S}iki\'c, Weiss, and Xiao (PSWX) and produce an analog of
Naimark's theorem for these wavelets at the level of their scaling functions.
We aim to make this discussion as self-contained as possible and provide a
different point of view on Parseval frame MRA wavelets than that of PSWX.Comment: 19 page
Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. XI: Stabilizing neutron stars against a ferromagnetic collapse
We construct a new Hartree-Fock-Bogoliubov (HFB) mass model, labeled HFB-18,
with a generalized Skyrme force. The additional terms that we have introduced
into the force are density-dependent generalizations of the usual and
terms, and are chosen in such a way as to avoid the high-density
ferromagnetic instability of neutron stars that is a general feature of
conventional Skyrme forces, and in particular of the Skyrme forces underlying
all the HFB mass models that we have developed in the past. The remaining
parameters of the model are then fitted to essentially all the available mass
data, an rms deviation of 0.585 MeV being obtained. The new model thus
gives almost as good a mass fit as our best-fit model HFB-17 ( = 0.581
MeV), and has the advantage of avoiding the ferromagnetic collapse of neutron
stars.Comment: accepted for publication in Physical Review
Dipetalonema viteae (Filarioidea): development of the infective larvae in micropore chambers implanted into normal, infected and immunized jirds
Transverse target spin asymmetry in inclusive DIS with two-photon exchange
We study the transverse target spin dependence of the cross section for
inclusive electron-nucleon scattering with unpolarized beam. Such dependence is
absent in the one-photon exchange approximation (Christ-Lee theorem) and arises
only in higher orders of the QED expansion, from the interference of one-photon
and absorptive two-photon exchange amplitudes as well as from real photon
emission (bremsstrahlung). We demonstrate that the transverse spin-dependent
two-photon exchange cross section is free of QED infrared and collinear
divergences. We argue that in DIS kinematics the transverse spin dependence
should be governed by a "parton-like" mechanism in which the two-photon
exchange couples mainly to a single quark. We calculate the normal spin
asymmetry in an approximation where the dominant contribution arises from quark
helicity flip due to interactions with non-perturbative vacuum fields
(constituent quark picture) and is proportional to the quark transversity
distribution in the nucleon. Such helicity-flip processes are not significantly
Sudakov-suppressed if the infrared scale for gluon emission in the photon-quark
subprocess is of the order of the chiral symmetry breaking scale, mu_chiral^2
>> Lambda_QCD^2. We estimate the asymmetry in the kinematics of the planned
Jefferson Lab Hall A experiment to be of the order 10^{-4}, with different sign
for proton and neutron. We also comment on the spin dependence in the limit of
soft high-energy scattering.Comment: 22 pages, 14 figures; uses revtex
Convective intensification of magnetic fields in the quiet Sun
Kilogauss-strength magnetic fields are often observed in intergranular lanes at the photosphere in the quiet Sun. Such fields are stronger than the equipartition field B_e, corresponding to a magnetic energy density that matches the kinetic energy density of photospheric convection, and comparable with the field B_p that exerts a magnetic pressure equal to the ambient gas pressure. We present an idealised numerical model of three-dimensional compressible magnetoconvection at the photosphere, for a range of values of the magnetic Reynolds number. In the absence of a magnetic field, the convection is highly supercritical and is characterised by a pattern of vigorous, time-dependent, “granular” motions. When a weak magnetic field is imposed upon the convection, magnetic flux is swept into the convective downflows where it forms localised concentrations. Unless this process is significantly inhibited by magnetic diffusion, the resulting fields are often much greater than B_e, and the high magnetic pressure in these flux elements leads to their being partially evacuated. Some of these flux elements contain ultra-intense magnetic fields that are significantly greater than B_p. Such fields are contained by a combination of the thermal pressure of the gas and the dynamic pressure of the convective motion, and they are constantly evolving. These ultra-intense fields develop owing to nonlinear interactions between magnetic fields and convection; they cannot be explained in terms of “convective collapse” within a thin flux tube that remains in overall pressure equilibrium with its surroundings
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