3,049 research outputs found
Can Self-Organizing Maps accurately predict photometric redshifts?
We present an unsupervised machine learning approach that can be employed for
estimating photometric redshifts. The proposed method is based on a vector
quantization approach called Self--Organizing Mapping (SOM). A variety of
photometrically derived input values were utilized from the Sloan Digital Sky
Survey's Main Galaxy Sample, Luminous Red Galaxy, and Quasar samples along with
the PHAT0 data set from the PHoto-z Accuracy Testing project. Regression
results obtained with this new approach were evaluated in terms of root mean
square error (RMSE) to estimate the accuracy of the photometric redshift
estimates. The results demonstrate competitive RMSE and outlier percentages
when compared with several other popular approaches such as Artificial Neural
Networks and Gaussian Process Regression. SOM RMSE--results (using
z=z--z) for the Main Galaxy Sample are 0.023, for the
Luminous Red Galaxy sample 0.027, Quasars are 0.418, and PHAT0 synthetic data
are 0.022. The results demonstrate that there are non--unique solutions for
estimating SOM RMSEs. Further research is needed in order to find more robust
estimation techniques using SOMs, but the results herein are a positive
indication of their capabilities when compared with other well-known methods.Comment: 5 pages, 3 figures, submitted to PAS
Automotive technology status and projections. Volume 2: Assessment report
Current and advanced conventional engines, advanced alternative engines, advanced power train components, and other energy conserving automobile modifications which could be implemented by the end of this century are examined. Topics covered include gas turbine engines, Stirling engines, advanced automatic transmissions, alternative fuels, and metal and ceramic technology. Critical problems are examined and areas for future research are indicated
Automotive technology status and projections. Volume 1: Executive summary
Fuel economy, exhaust emissions, multifuel capability, advanced materials and cost/manufacturability for both conventional and advanced alternative power systems were assessed. To insure valid comparisons of vehicles with alternative power systems, the concept of an Otto-Engine-Equivalent (OEE) vehicle was utilized. Each engine type was sized to provide equivalent vehicle performance. Sensitivity to different performance criteria was evaluated. Fuel economy projections are made for each engine type considering both the legislated emission standards and possible future emissions requirements
The S-matrix of the Faddeev-Reshetikhin Model, Diagonalizability and PT Symmetry
We study the question of diagonalizability of the Hamiltonian for the
Faddeev-Reshetikhin (FR) model in the two particle sector. Although the two
particle S-matrix element for the FR model, which may be relevant for the
quantization of strings on , has been calculated recently
using field theoretic methods, we find that the Hamiltonian for the system in
this sector is not diagonalizable. We trace the difficulty to the fact that the
interaction term in the Hamiltonian violating Lorentz invariance leads to
discontinuity conditions (matching conditions) that cannot be satisfied. We
determine the most general quartic interaction Hamiltonian that can be
diagonalized. This includes the bosonic Thirring model as well as the bosonic
chiral Gross-Neveu model which we find share the same S-matrix. We explain this
by showing, through a Fierz transformation, that these two models are in fact
equivalent. In addition, we find a general quartic interaction Hamiltonian,
violating Lorentz invariance, that can be diagonalized with the same two
particle S-matrix element as calculated by Klose and Zarembo for the FR model.
This family of generalized interaction Hamiltonians is not Hermitian, but is
symmetric. We show that the wave functions for this system are also
symmetric. Thus, the theory is in a unbroken phase which guarantees the
reality of the energy spectrum as well as the unitarity of the S-matrix.Comment: 32 pages, 1 figure; references added, version published in JHE
On the breakdown of perturbative integrability in large N matrix models
We study the perturbative integrability of the planar sector of a massive
SU(N) matrix quantum mechanical theory with global SO(6) invariance and
Yang-Mills-like interaction. This model arises as a consistent truncation of
maximally supersymmetric Yang-Mills theory on a three-sphere to the lowest
modes of the scalar fields. In fact, our studies mimic the current
investigations concerning the integrability properties of this gauge theory.
Like in the field theory we can prove the planar integrability of the SO(6)
model at first perturbative order. At higher orders we restrict ourselves to
the widely studied SU(2) subsector spanned by two complexified scalar fields of
the theory. We show that our toy model satisfies all commonly studied
integrability requirements such as degeneracies in the spectrum, existence of
conserved charges and factorized scattering up to third perturbative order.
These are the same qualitative features as the ones found in super Yang-Mills
theory, which were enough to conjecture the all-loop integrability of that
theory. For the SO(6) model, however, we show that these properties are not
sufficient to predict higher loop integrability. In fact, we explicitly
demonstrate the breakdown of perturbative integrability at fourth order.Comment: 27 page
Another short-burst host galaxy with an optically obscured high star formation rate: The case of GRB 071227
We report on radio continuum observations of the host galaxy of the short
gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array
(ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux
density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured
star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was
found from optical emission lines. Among the ~30 well-identified and studied
host galaxies of short bursts this is the third case where the host is found to
undergo an episode of intense star formation. This suggests that a fraction of
all short-burst progenitors hosted in star-forming galaxies could be physically
related to recent star formation activity, implying a relatively short merger
time scale.Comment: 6 pages, ApJ, accepted for publicatio
The Zamolodchikov-Faddeev algebra for open strings attached to giant gravitons
We extend the Zamolodchikov-Faddeev algebra for the superstring sigma model
on , which was formulated by Arutyunov, Frolov and
Zamaklar, to the case of open strings attached to maximal giant gravitons,
which was recently considered by Hofman and Maldacena. We obtain boundary
-matrices which satisfy the standard boundary Yang-Baxter equation.Comment: 22 pages, no figure; added a referenc
The Ising model and planar N=4 Yang-Mills
The scattering-matrix for planar Yang-Mills with N=4 supersymmetry relies on
the assumption that integrability holds to all orders in perturbation theory.
In this note we define a map from the spectral variables x^{\pm},
parameterizing the long-range magnon momenta, to couplings in a two-dimensional
Ising model. Under this map integrability of planar N=4 Yang-Mills becomes
equivalent to the Yang-Baxter equation for the two-dimensional Ising model, and
the long-range variables x^{\pm} translate into the entries of the Ising
transfer matrices. We explore the Ising correlation length which equals the
inverse magnon momentum in the small momentum limit. The critical regime is
thus reached for vanishing magnon momentum. We also discuss the meaning of the
Kramers-Wannier duality transformation on the gauge theory, together with that
of the Ising model critical points.Comment: 24 pages. v2: References added and minor typos correcte
Unsteady Force Measurements On Fully Wetted Hydrofoils in Heaving Motion
An experimental investigation is reported of the unsteady forces due to heaving motion of fully wetted hydrofoils of unity aspect ratio and also in two-dimensional flow. The tests covered a broad range of reduced frequency and determined the effects of variation in submergence depth, angle of attack, oscillation amplitude, and flow velocity. In general, the findings agree well with available theoretical calculations, but some unexpected variations were found for the case of a wedge-shaped foil and for changes in angle of attack
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