4,804 research outputs found
On lunar ''temperatures''
Thermal measurements on lunar surface at radio wavelength
Numerical Implementation of Gradient Algorithms
A numerical method for computational implementation of gradient dynamical systems is presented. The method is based upon the development of geometric integration numerical methods, which aim at preserving the dynamical properties of the original ordinary differential
equation under discretization. In particular, the proposed method belongs to the class of discrete gradients methods, which substitute the gradient of the continuous equation with a discrete gradient, leading to a map that possesses the same Lyapunov function of the dynamical system,
thus preserving the qualitative properties regardless of the step size. In this work, we apply a discrete gradient method to the implementation of Hopfield neural networks. Contrary to most geometric integration
methods, the proposed algorithm can be rewritten in explicit form, which considerably improves its performance and stability. Simulation results show that the preservation of the Lyapunov function leads to an improved performance, compared to the conventional discretization.Spanish Government project no. TIN2010-16556 Junta de Andalucía project no. P08-TIC-04026 Agencia Española de Cooperación Internacional
para el Desarrollo project no. A2/038418/1
Tropospheric range error parameters: Further studies
Improved parameters are presented for predicting the tropospheric effect on electromagnetic range measurements from surface meteorological data. More geographic locations have been added to the earlier list. Parameters are given for computing the dry component of the zenith radio range effect from surface pressure alone with an rms error of 1 to 2 mm, or the total range effect from the dry and wet components of the surface refractivity and a two-part quartic profile model. The new parameters are obtained, as before, from meteorological balloon data but with improved procedures, including the conversion of the geopotential heights of the balloon data to actual or geometric heights before using the data. The revised values of the parameter k (dry component of vertical radio range effect per unit pressure at the surface) show more latitude variation than is accounted for by the variation of g, the acceleration of gravity
Dense Associative Memory is Robust to Adversarial Inputs
Deep neural networks (DNN) trained in a supervised way suffer from two known
problems. First, the minima of the objective function used in learning
correspond to data points (also known as rubbish examples or fooling images)
that lack semantic similarity with the training data. Second, a clean input can
be changed by a small, and often imperceptible for human vision, perturbation,
so that the resulting deformed input is misclassified by the network. These
findings emphasize the differences between the ways DNN and humans classify
patterns, and raise a question of designing learning algorithms that more
accurately mimic human perception compared to the existing methods.
Our paper examines these questions within the framework of Dense Associative
Memory (DAM) models. These models are defined by the energy function, with
higher order (higher than quadratic) interactions between the neurons. We show
that in the limit when the power of the interaction vertex in the energy
function is sufficiently large, these models have the following three
properties. First, the minima of the objective function are free from rubbish
images, so that each minimum is a semantically meaningful pattern. Second,
artificial patterns poised precisely at the decision boundary look ambiguous to
human subjects and share aspects of both classes that are separated by that
decision boundary. Third, adversarial images constructed by models with small
power of the interaction vertex, which are equivalent to DNN with rectified
linear units (ReLU), fail to transfer to and fool the models with higher order
interactions. This opens up a possibility to use higher order models for
detecting and stopping malicious adversarial attacks. The presented results
suggest that DAM with higher order energy functions are closer to human visual
perception than DNN with ReLUs
An Information-Based Neural Approach to Constraint Satisfaction
A novel artificial neural network approach to constraint satisfaction
problems is presented. Based on information-theoretical considerations, it
differs from a conventional mean-field approach in the form of the resulting
free energy. The method, implemented as an annealing algorithm, is numerically
explored on a testbed of K-SAT problems. The performance shows a dramatic
improvement to that of a conventional mean-field approach, and is comparable to
that of a state-of-the-art dedicated heuristic (Gsat+Walk). The real strength
of the method, however, lies in its generality -- with minor modifications it
is applicable to arbitrary types of discrete constraint satisfaction problems.Comment: 13 pages, 3 figures,(to appear in Neural Computation
Earthquake cycles and neural reverberations
Driven systems of interconnected blocks with stick-slip friction capture main features of earthquake processes. The microscopic dynamics closely resemble those of spiking nerve cells. We analyze the differences in the collective behavior and introduce a class of solvable models. We prove that the models exhibit rapid phase locking, a phenomenon of particular interest to both geophysics and neurobiology. We study the dependence upon initial conditions and system parameters, and discuss implications for earthquake modeling and neural computation
Neural network computation by in vitro transcriptional circuits
The structural similarity of neural networks and genetic regulatory networks
to digital circuits, and hence to each other, was noted from the
very beginning of their study [1, 2]. In this work, we propose a simple
biochemical system whose architecture mimics that of genetic regulation
and whose components allow for in vitro implementation of arbitrary
circuits. We use only two enzymes in addition to DNA and RNA
molecules: RNA polymerase (RNAP) and ribonuclease (RNase). We
develop a rate equation for in vitro transcriptional networks, and derive
a correspondence with general neural network rate equations [3].
As proof-of-principle demonstrations, an associative memory task and a
feedforward network computation are shown by simulation. A difference
between the neural network and biochemical models is also highlighted:
global coupling of rate equations through enzyme saturation can lead
to global feedback regulation, thus allowing a simple network without
explicit mutual inhibition to perform the winner-take-all computation.
Thus, the full complexity of the cell is not necessary for biochemical
computation: a wide range of functional behaviors can be achieved with
a small set of biochemical components
Exciton-polariton behaviour in bulk and polycrystalline ZnO
We report detailed reflectance studies of the exciton–polariton structure of thin film polycrystalline ZnO
and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are
fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline
ZnO differ substantially from the bulk material, with Fabry–Perot oscillations at energies below the transverse A
exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no
evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the
strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping
such that the Fabry–Perot modes are eliminated. Good qualitative agreement is achieved between the model and
data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly
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