Stark-Wannier states and Bender-Wu singularities

A new angular velocity observer for attitude tracking of spacecraft

This article explores the exponential stabilization issue of a class of state-based switched inertial complex-valued neural networks with multiple delays via event-triggered control. First, the state-based switched inertial complex-valued neural networks with multiple delays are modeled. Second, by separating the real and imaginary parts of complex values, the state-based switched inertial complex-valued neural networks are transformed into two state-based switched inertial real-valued neural networks. Through the variable substitution method, the model of the second-order inertial neural networks is transformed into a model of the first-order neural networks. Third, an event-triggered controller with the transmission sequence is designed to study the exponential stabilization issue of neural networks constructed above. Then, by constructing the Lyapunov functions and based on some inequalities, we obtain sufficient conditions for exponential stabilization of the proposed neural networks. Furthermore, it is proved that the Zeno phenomenon cannot happen under the designed event-triggered controller. Finally, a simulation example is given to illustrate the correctness of the results.

We study the problem of approximating the value of the matching polynomial on graphs with edge parameter γ, where γ takes arbitrary values in the complex plane. When γ is a positive real, Jerrum and Sinclair showed that the problem admits an FPRAS on general graphs. For general complex values of γ, Patel and Regts, building on methods developed by Barvinok, showed that the problem admits an FPTAS on graphs of maximum degree Δ as long as γ is not a negative real number less than or equal to −1/(4(Δ −1)). Our first main result completes the picture for the approximability of the matching polynomial on bounded degree graphs. We show that for all Δ ≥ 3 and all real γ less than −1/(4(Δ −1)), the problem of approximating the value of the matching polynomial on graphs of maximum degree Δ with edge parameter γ is #P-hard. We then explore whether the maximum degree parameter can be replaced by the connective constant. Sinclair et al. showed that for positive real γ, it is possible to approximate the value of the matching polynomial using a correlation decay algorithm on graphs with bounded connective constant (and potentially unbounded maximum degree). We first show that this result does not extend in general in the complex plane; in particular, the problem is #P-hard on graphs with bounded connective constant for a dense set of γ values on the negative real axis. Nevertheless, we show that the result does extend for any complex value γ that does not lie on the negative real axis. Our analysis accounts for complex values of γ using geodesic distances in the complex plane in the metric defined by an appropriate density function.

Edge and corner states in two-dimensional finite phononic crystals: Simulation and experimental study

The authors show that the density of states for an electron in a finite periodic lattice and an external electric field has a background of states proportional to Ed/2, where E and d are the energy and dimensionality of the system, respectively. This background is modulated by an asymptotically periodic structure with the Stark period that involves a fraction of order E-d/2 of the total number of states in each period. The structure is more pronounced in lower-dimensionality systems. Numerical results are included for a superlattice with a saw-tooth potential.

A class of continuous robust controllers termed Robust Integral of the Sign of the Error (RISE) have been published over the past two decades as a means to yield asymptotic tracking error convergence and asymptotic identification of time-varying uncertainties, for classes of nonlinear systems that are subject to sufficiently smooth bounded exogenous disturbances and/or modeling uncertainties. Despite the wide application of RISE-based techniques, an open question that has eluded researchers during this time-span is whether the asymptotic tracking error convergence is also uniform or exponential. This question has remained open due to certain limitations in the traditional construction of a Lyapunov function for RISE-based error systems. In this letter, new insights on the construction of a Lyapunov function are used that result in an exponential stability result for RISE-based controllers. As an outcome of this breakthrough, the inherent learning capability of RISE-based controllers is shown to yield exponential identification of state-dependent disturbances/uncertainty.

In this paper we obtain large $z$ asymptotic expansions in the complex plane for the tau function corresponding to special function solutions of the Painlev\'e II differential equation. Using the fact that these tau functions can be written as $n\times n$ Wronskian determinants involving classical Airy functions, we use Heine's formula to rewrite them as $n$-fold integrals, which can be asymptotically approximated using the classical method of steepest descent in the complex plane.

The Stokes phenomenon refers to the emergence of jump discontinuities in asymptotic expansions at specific rays in the complex plane. This book presents a radical theory for the phenomenon by introducing the concept of regularization. Two methods of regularization, Borel summation and Mellin-Barnes regularization, are used to derive general expressions for the regularized values of asymptotic expansions throughout the complex plane. Though different, both yield identical values, which, where possible, agree with the original functions. Consequently, asymptotics has been elevated to a true discipline yielding precise solutions. All researchers, who seek asymptotic solutions to problems, will find this a most valuable book.

This article is concerned with the asymptotic behaviour of m(λ), the Titchmarsh-Weyl m-coefficient, for the singular eigenvalue equation y“ + (λ − q(x))y = 0 on [0, ∞), as λ →∞ in a sector in the upper half of the complex plane. It is assumed that the potential function q is integrable near 0. A simplified proof is given of a result of Atkinson [7], who derived the first two terms in the asymptotic expansion of m(λ), and a sharper error bound is obtained. Theproof is then generalised to derive subsequent terms in the asymptotic expansion. It is shown that the Titchmarsh-Weyl m-coefficient admits an asymptotic power series expansion if the potential function satisfies some smoothness condition. A simple method to compute the expansion coefficients is presented. The results for the first few coefficients agree with those given by Harris [9].

A central issue in hadron spectroscopy is to deduce --- and interpret --- resonance parameters, namely pole positions and residues, from experimental data, for those are the quantities to be compared to lattice QCD or model calculations. However, not every structure in the observables derives from a resonance pole: the origin might as well be branch points, either located on the real axis (when a new channel comprised of stable particles opens) or in the complex plane (when at least one of the intermediate particles is unstable). In this paper we demonstrate first the existence of such branch points in the complex plane and then show on the example of the piN P11 partial wave that it is not possible to distinguish the structures induced by the latter from a true pole signal based on elastic data alone.

In this paper, the photonic band diagram and the density of states in finite photonic structures are analyzed and optimized for broad bandwidths using the apodization method. Based on the effective propagation constant, the group refractive index can be computed for the finite structures, which is adopted to reveal the physical explanation on the optimal band gaps. The surface states of the finite photonic crystal and the mode matching at the interfaces are also considered. It is found that the high group index of the photonic structures can obtain broad bandwidths and also enhance the mode matching at the interfaces. By slowly varying the radii of the cylinders monotonously in two dimensions, the first band gap can be increased from 10.6% to 20.04%. This work provides a useful tool for the optimal design of finite photonic crystals.

Among all bifurcation behaviors of analytic parametric families of real planar vector fields, those that stand out most prominently are confluences of distinct stationary points. The qualitative change is so drastic that in some classes of families (e.g., fold-like bifurcations) the stationary points leave the real plane altogether and slip into the complex plane. Although they disappear from the real domain they continue to organize the dynamics, and studying complex planar vector fields becomes a necessity even for real bifurcations.Our main concern is to describe a la Martinet-Ramis the analytical classification of generic holomorphic families unfolding a saddle-node vector field, and to relate this classification both to the dynamics of individual members of the family and to analytic properties of the saddle-node. For instance the problem of the existence of an analytic center-manifold for the saddle-node is characterized in terms of persistence (as the parameter tends to the bifurcation value) of heteroclinic connections between stationary points.We emphasize the geometric aspect of the classification. Complex trajectories are connected real surfaces allowing for richer geometric constructions as compared to 1-dimensional real trajectories. The trajectories are split by a finite collection of open “fibred squid sectors,” attached by spirals to stationary points within their adherence. The sectors are carved in such a way that one can construct an analytic and bounded conjugacy between the vector field and its formal normal form. The invariants of classification are obtained as transition maps of overlapping such normalization charts. Since we can perform this sectorial normalization analytically in the parameter, by restricting its values to “cells” covering the parameter space minus the bifurcation value, the resulting finite collection of functional invariants is analytic on parameter cells and continuous on their adherence. In that sense it “unfolds” Martinet-Ramis invariant of the saddle-node.The inverse problem (or realization) is addressed in the case of a persistent heteroclinic connections and provides unique normal forms (universal family for the analytic classification). We particularly show that in general the invariant cannot depend holomorphically on the parameter over a full neighborhood of the bifurcation value.

A complex fuzzy set is a set whose membership grades are complex values in the unit circle in the complex plane. This paper introduces the concept of approximate parallelity between complex fuzzy sets based on the phase of complex-valued membership grade. After that, the property of approximate parallelity preserving for complex fuzzy operators and complex fuzzy inference are investigated.

If we associate each point z of some domain D of the z-plane with a certain complex value w according to a given law then w is called a function of z. Two geometrical interpretations of the functional relation are particularly useful. One uses one plane, the other two planes. The value w belonging to the point z (or, if more expedient, \(\bar w \) can be thought of as a vector acting on the point z; in this way a vector field is defined in the domain D. In the other interpretation, the value w associated with the point z in the z-plane is conceived as a point in another complex plane (w-plane). In this way the domain D is mapped onto a certain point set of the w-plane.

We derive uniform asymptotic expansions for polynomials orthogonal with respect to a class of weight functions that are real analytic and behave asymptotically like the Freud weight at infinity. Although the limiting zero distributions are the same as in the Freud cases, the asymptotic expansions are different due to the fact that the weight functions may have a finite or infinite number of zeros on the imaginary axis. To resolve the singularities caused by these zeros, an auxiliary function is introduced in the Riemann–Hilbert analysis. Asymptotic formulas are established in several regions covering the whole complex plane. We take the continuous dual Hahn polynomials as an example to illustrate our main results. Some numerical verifications are also given.