Multidimensional Gaussian probability density and its applications in the degenerate case

In this paper we consider a geometric viewpoint to analyze the behaviour of the Conjugate Gradient (CG) method, for the solution of a symmetric linear system, when at current step a pivot breakdown possibly occurs (degenerate case). As well known this can occur when the system matrix is indefinite or singular. In the latter case the CG gets stuck, since the steplength along the current search direction cannot be computed. We show here that a simple geometric interpretation can be provided for the degenerate case, as long as some basics on projective geometry in the Euclidean space are considered.

A Superconducting QUantum Interference Device (SQUID) modulated by a fast oscillating magnetic flux can be used as a parametric amplifier, providing gain with very little added noise. Here, we develop linearized models to describe the parametrically flux-pumped SQUID in terms of an impedance. An unpumped SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped SQUID develops an additional, parallel element which we have coined the ``pumpistor.'' Parametric gain can be understood as a result of a negative resistance of the pumpistor. In the degenerate case, the gain is sensitive to the relative phase between the pump and signal. In the nondegenerate case, gain is independent of this phase. We develop our models first for degenerate parametric pumping in the three-wave and four-wave cases, where the pump frequency is either twice or equal to the signal frequency, respectively. We then derive expressions for the nondegenerate case where the pump frequency is not a multiple of the signal frequency, where it becomes necessary to consider idler tones which develop. For the nondegenerate three-wave case, we present an intuitive picture for a parametric amplifier containing a flux-pumped SQUID where current at the signal frequency depends upon the load impedance at an idler frequency. This understanding provides insight and readily testable predictions of circuits containing flux-pumped SQUIDs.

This paper deals with multichannel time series, and extends the autocorrelation method for fitting the time series from the nondegenerate case to the general case. We shall show that the AR model obtained by solving the normal equations is always stable even in the degenerate case. The multichannel Levinson algorithm for solving the normal equations is extended from the nondegenerate case to the general case. In the nondegenerate case, the first n+1 values of the auto-correlation sequence of the obtained AR model of order n are identical with those of the original data. The next considered is the extrapolation problem of an autocorrelation sequence, that is, given data <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">{R_{0}, R_{1},..., R_{n}}</tex> , it is to construct a model whose first n+1 values of the autocorrelation sequence are identical with these data. We shall show that, even in the degenerate case, when the given data <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">{R_{0}, R_{1},..., R_{n}}</tex> are taken from the autocorrelation sequence of a purely nondeterministic time series, they are identical with the first n+1 values of the autocorrelation sequence of the AR model of order n.

Tomography vs quantum control for a three-level atom

We prove the existence of multidimensional traveling-wave solutions to the scalar equation for the transport of solutes (contaminants) with nonlinear adsorption and spatially periodic convection-diffusion-adsorption coefficients under the assumption that the nonlinear adsorption function satisfies the Lax and Oleinik entropy conditions. In the nondegenerate case, we also prove the uniqueness of the traveling waves. These traveling waves are analogues of viscous shock profiles. They propagate with effective speeds that depend on the periodic porous media only up to their mean states, and are given by an averaged Rankine-Hugoniot relation. This is a direct consequence of the fact that the transport equation is in conservation form. We use the sliding domain method, the continuation method, spectral theory, maximum principles, and a priori estimates. In the degenerate case, the traveling waves are weak solutions of a degenerate parabolic equation and are only Holder continuous. We obtain them by taking suitable limits on the non-degenerate traveling waves. The uniqueness of the degenerate traveling waves is open.

Asymptotic spreading of a time periodic diffusion equation with degenerate monostable nonlinearity

The compartmentalization of distinct templates in protocells and the exchange of templates between them (migration) are key elements of a modern scenario for prebiotic evolution. Here we use the diffusion approximation of population genetics to study analytically the steady-state properties of such a prebiotic scenario. The coexistence of distinct template types inside a protocell is achieved by a selective pressure at the protocell level (group selection) favoring protocells with a mixed template composition. In the degenerate case, where the templates have the same replication rate, we find that a vanishingly small migration rate suffices to eliminate the segregation effect of random drift and so to promote coexistence. In the nondegenerate case, a small migration rate greatly boosts coexistence as compared with the situation where there is no migration. However, increase of the migration rate beyond a critical value leads to the complete dominance of the more efficient template type (homogeneous regime). In this case, we find a continuous phase transition separating the homogeneous and the coexistence regimes, with the order parameter vanishing linearly with the distance to the transition point.

We investigate the dynamics of the wave packet formed by two codirected strongly interacting waves propagating in a medium with cubic nonlinearity. We obtain a soliton solution of the nonlinear Schrodinger equation in the degenerate case where the wave packet is described by a single partial momentum. In the nondegenerate case, we use the variational method to find the equation for the pulse duration, which turns out to be analogous to the equation for the coordinate in the Kepler problem. Solving it, we find the dependences of the pulse duration on the propagation distance in the cases of “finite” and “infinite” propagation regimes.

We are concerned with the problem of computing L((p). REMARK. Let G be a compact Lie group acting on X as a group of holomorphic diffeomorphisms and cp e G. The problem in this case has been solved by Atiyah and Singer, see [2]. Also in the case cp has isolated fixed points, the problem was solved in the nondegenerate case (see §2 for definition) by Atiyah and Bott in [1] and by Toledo and Tong ii>[6] and [7] in the degenerate case.

We propose a finite volume scheme for convection–diffusion equations with nonlinear diffusion. Such equations arise in numerous physical contexts. We will particularly focus on the drift-diffusion system for semiconductors and the porous media equation. In these two cases, it is shown that the transient solution converges to a steady-state solution as t tends to infinity. The introduced scheme is an extension of the Scharfetter–Gummel scheme for nonlinear diffusion. It remains valid in the degenerate case and preserves steady-states. We prove the convergence of the scheme in the nondegenerate case. Finally, we present some numerical simulations applied to the two physical models introduced and we underline the efficiency of the scheme to preserve long-time behavior of the solutions.

In the calculation of quantum systems in most cases, the perturbation theory is used. Lot of calculation techniques are used for both (nondegenerate and degenerate) cases. In our contribution we evaluate the projection operator technique in such case when the Hamiltonian of the given quantum system has point spectrum with finite degree of degeneracy.

Optical systems expressed in terms of fractional Fourier transforms

Formulae with matrix elements regarding screened polar optical (PO), deformation acoustic (DA), and impurity (I) scattering, nonparabolicity, and s‐p interaction of the conduction band for nondegenerate and degenerate cases are used for the calculation of the mean energy 〈ϵ〉 of hot electrons in n‐type InSb semiconductors. The distribution function (DF) taken as an unexpended diplaced one, due to the lower values of Te, results in an electron‐gas “cooling” in nondegenerate n‐type InSb below To = 80 K, as well as in a reduction of the electron mobility in comparison with the calculated one for expanded DF. Taking into account the interaction of C‐ and V‐zones limits the runaway of electrons. A strong increase of the electric field increases the mean energy of carriers, being more effective for high‐resistance specimens due to the increase of the frequency of inter‐electron collisions. The mean energy of carriers calculated is compared with data of experiments performed in a wide range of electron concentrations.

The critical temperatures of the Ising model can be obtained by considering the elementary cells of the corresponding lattice, namely the square lattice in two dimensions and the cube in three dimensions. The configurations in the statistical sum of a cell are divided into nondegenerate and degenerate cases. At the critical temperature of the infinite lattice the contributions of these two groups of configurations are assumed to be equal. This conjecture reproduces the exact Onsager result for two dimensions and the numerical result for the three-dimensional Ising lattices. Although this conjecture is not exact, it gives insight into the nature of the transitions.

This paper exploits a general approach to geometric optics in inhomogeneous plasmas based on the properties of the local dielectric tensor ?. We express ? in terms of its eigenvalues ?j and eigenvectors ?. Then to zeroth order in the geometric optics approximation the determinant D = ?1?2?3 vanishes and the elements ?j vanish separately in pairs or simultaneously. It is shown that this branching in the dispersion relation changes the formulation of the geometric optics equations. The ray tracing and the transport of the amplitude of the wave in both degenerate and nondegenerate cases is described. The general procedure for transition through a boundary between degenerate and nondegenerate regions, where the rays split into two parts each following a different branch of the dispersion relation is also presented in this paper. We demonstrate our general method in a case, where radiation from a vacuum region enters an inhomogeneous magnetized plasma layer.