2,334 research outputs found
Spectral Norm Regularization for Improving the Generalizability of Deep Learning
We investigate the generalizability of deep learning based on the sensitivity
to input perturbation. We hypothesize that the high sensitivity to the
perturbation of data degrades the performance on it. To reduce the sensitivity
to perturbation, we propose a simple and effective regularization method,
referred to as spectral norm regularization, which penalizes the high spectral
norm of weight matrices in neural networks. We provide supportive evidence for
the abovementioned hypothesis by experimentally confirming that the models
trained using spectral norm regularization exhibit better generalizability than
other baseline methods
Pulse-phase resolved spectroscopy of continuum and reflection in SAX J1808.4-3658
We perform phase-resolved spectroscopy of the accreting millisecond pulsar,
SAX J1808.4-3658, during the slow-decay phase of the 2002 outburst. Simple
phenomenological fits to RXTE PCA data reveal a pulsation in the iron line at
the spin frequency of the neutron star. However, fitting more complex spectral
models reveals a degeneracy between iron-line pulsations and changes in the
underlying hotspot blackbody temperature with phase. By comparing with the
variations in reflection continuum, which are much weaker than the iron line
variations, we infer that the iron-line is not pulsed. The observed spectral
variations can be explained by variations in blackbody temperature associated
with rotational Doppler shifts at the neutron star surface. By allowing
blackbody temperature to vary in this way, we also find a larger phase-shift
between the pulsations in the Comptonised and blackbody components than has
been seen in previous work. The phase-shift between the pulsation in the
blackbody temperature and normalisation is consistent with a simple model where
the Doppler shift is maximised at the limb of the neutron star, ~90 degrees
prior to maximisation of the hot-spot projected area.Comment: 8 pages, 10 figures, 2 tables. Accepted by MNRA
Universal transient behavior in large dynamical systems on networks
We analyze how the transient dynamics of large dynamical systems in the
vicinity of a stationary point, modeled by a set of randomly coupled linear
differential equations, depends on the network topology. We characterize the
transient response of a system through the evolution in time of the squared
norm of the state vector, which is averaged over different realizations of the
initial perturbation. We develop a mathematical formalism that computes this
quantity for graphs that are locally tree-like. We show that for unidirectional
networks the theory simplifies and general analytical results can be derived.
For example, we derive analytical expressions for the average squared norm for
random directed graphs with a prescribed degree distribution. These analytical
results reveal that unidirectional systems exhibit a high degree of
universality in the sense that the average squared norm only depends on a
single parameter encoding the average interaction strength between the
individual constituents. In addition, we derive analytical expressions for the
average squared norm for unidirectional systems with fixed diagonal disorder
and with bimodal diagonal disorder. We illustrate these results with numerical
experiments on large random graphs and on real-world networks.Comment: 19 pages, 7 figures. Substantially enlarged version. Submitted to
Physical Review Researc
Linear Prediction of Long-Range Dependent Time Series
We present two approaches for next step linear prediction of long memory time
series. The first is based on the truncation of the Wiener-Kolmogorov predictor
by restricting the observations to the last terms, which are the only
available values in practice. Part of the mean squared prediction error comes
from the truncation, and another part comes from the parametric estimation of
the parameters of the predictor. By contrast, the second approach is
non-parametric. An AR() model is fitted to the long memory time series and
we study the error made with this misspecified model
A new formalism for the estimation of the CP-violation parameters
In this paper, we use the time super-operator formalism in the 2-level
Friedrichs model \cite{fried} to obtain a phenomenological model of mesons
decay. Our approach provides a fairly good estimation of the CP symmetry
violation parameter in the case of K, B and D mesons. We also propose a crucial
test aimed at discriminating between the standard approach and the time
super-operator approach developed throughout the paper
Finding Structure with Randomness: Probabilistic Algorithms for Constructing Approximate Matrix Decompositions
Low-rank matrix approximations, such as the truncated singular value decomposition and the rank-revealing QR decomposition, play a central role in data analysis and scientific computing. This work surveys and extends recent research which demonstrates that randomization offers a powerful tool for performing low-rank matrix approximation. These techniques exploit modern computational architectures more fully than classical methods and open the possibility of dealing with truly massive data sets. This paper presents a modular framework for constructing randomized algorithms that compute partial matrix decompositions. These methods use random sampling to identify a subspace that captures most of the action of a matrix. The input matrix is then compressed—either explicitly or
implicitly—to this subspace, and the reduced matrix is manipulated deterministically to obtain the desired low-rank factorization. In many cases, this approach beats its classical competitors in terms of accuracy, robustness, and/or speed. These claims are supported by extensive numerical experiments and a detailed error analysis. The specific benefits of randomized techniques depend on the computational environment. Consider the model problem of finding the k dominant components of the singular value decomposition of an m × n matrix. (i) For a dense input matrix, randomized algorithms require O(mn log(k))
floating-point operations (flops) in contrast to O(mnk) for classical algorithms. (ii) For a sparse input matrix, the flop count matches classical Krylov subspace methods, but the randomized approach is more robust and can easily be reorganized to exploit multiprocessor architectures. (iii) For a matrix that is too large to fit in fast memory, the randomized techniques require only a constant number of passes over the data, as opposed to O(k) passes for classical algorithms. In fact, it is sometimes possible to perform matrix approximation with a single pass over the data
Chandra observations of the recurrent nova CI Aql after its April 2000 outburst
We report the results of two Chandra observations of the recurrent nova CI
Aql at 14 and 16 months after its outburst in April 2000, respectively. The
X-ray emission is faint in both cases, without any noticeable change in
spectrum or intensity. Although the emission is very soft, it is not luminous
enough to be due to late-time H-burning. This implies that the luminous
supersoft phase ended even before the time predicted by the most recent
calculations. The details of the X-ray spectrum, together with the fact that
the observed X-ray intensity is brighter than pre-outburst (1992/1993), suggest
that the observed X-ray emission is either due to ionization of the
circumstellar material or due to the shocks within the wind and/or with the
surrounding medium.Comment: 10 pages ApJ style with 5 figures; accepted for publication in Ap
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