New articles on Nonlinear Sciences


[1] 2402.19426

Lévy noise-induced coherence resonance in the FitzHugh-Nagumo oscillator: numerical study versus experiment

Using a model of the FitzHugh-Nagumo oscillator in the excitable regime, we investigate the influence of the L\'evy noise's properties on the effect of coherence resonance. In particular, we demonstrate that the L\'evy noise can be a constructive or destructive factor providing for enhancement or suppression of noise-induced coherence. We show that the positive or negative role of the L\'evy noise impact is dictated by the noise's stability index and skewness parameter. The correlation time and the deviation of interspike intervals used in this analysis are shown to be maximized or minimized for an appropriate choice of the noise parameters. Numerical simulations are combined with experiments on an electronic circuit showing an excellent qualitative correspondence and proving thereby the robustness of the observed phenomena.


[2] 2402.18377

Out-of-Domain Generalization in Dynamical Systems Reconstruction

In science we are interested in finding the governing equations, the dynamical rules, underlying empirical phenomena. While traditionally scientific models are derived through cycles of human insight and experimentation, recently deep learning (DL) techniques have been advanced to reconstruct dynamical systems (DS) directly from time series data. State-of-the-art dynamical systems reconstruction (DSR) methods show promise in capturing invariant and long-term properties of observed DS, but their ability to generalize to unobserved domains remains an open challenge. Yet, this is a crucial property we would expect from any viable scientific theory. In this work, we provide a formal framework that addresses generalization in DSR. We explain why and how out-of-domain (OOD) generalization (OODG) in DSR profoundly differs from OODG considered elsewhere in machine learning. We introduce mathematical notions based on topological concepts and ergodic theory to formalize the idea of learnability of a DSR model. We formally prove that black-box DL techniques, without adequate structural priors, generally will not be able to learn a generalizing DSR model. We also show this empirically, considering major classes of DSR algorithms proposed so far, and illustrate where and why they fail to generalize across the whole phase space. Our study provides the first comprehensive mathematical treatment of OODG in DSR, and gives a deeper conceptual understanding of where the fundamental problems in OODG lie and how they could possibly be addressed in practice.


[3] 2402.18584

Adjusting Dynamics of Hopfield Neural Network via Time-variant Stimulus

As a paradigmatic model for nonlinear dynamics studies, the Hopfield Neural Network (HNN) demonstrates a high susceptibility to external disturbances owing to its intricate structure. This paper delves into the challenge of modulating HNN dynamics through time-variant stimuli. The effects of adjustments using two distinct types of time-variant stimuli, namely the Weight Matrix Stimulus (WMS) and the State Variable Stimulus (SVS), along with a Constant Stimulus (CS) are reported. The findings reveal that deploying four WMSs enables the HNN to generate either a four-scroll or a coexisting two-scroll attractor. When combined with one SVS, four WMSs can lead to the formation of an eight-scroll or four-scroll attractor, while the integration of four WMSs and multiple SVSs can induce grid-multi-scroll attractors. Moreover, the introduction of a CS and an SVS can significantly disrupt the dynamic behavior of the HNN. Consequently, suitable adjustment methods are crucial for enhancing the network's dynamics, whereas inappropriate applications can lead to the loss of its chaotic characteristics. To empirically validate these enhancement effects, the study employs an FPGA hardware platform. Subsequently, an image encryption scheme is designed to demonstrate the practical application benefits of the dynamically adjusted HNN in secure multimedia communication. This exploration into the dynamic modulation of HNN via time-variant stimuli offers insightful contributions to the advancement of secure communication technologies.


[4] 2402.18701

Nonlinear Defect Theory of Thermalization in Complex Multimoded Systems

We show that a single nonlinear defect can thermalize an initial excitation towards a Rayleigh-Jeans (RJ) state in complex multimoded systems. The thermalization can be hindered by disorder-induced localization phenomena which drive the system into a metastable RJ state. It involves only a (quasi-)isolated set of prethermal modes and can differ dramatically from the thermal RJ. We develop a one-parameter scaling theory that predicts the density of prethermal modes and we derive the modal relaxation rate distribution, establishing analogies with the Thouless conductance. Our results are relevant to photonics, optomechanics, and cold atoms.


[5] 2402.19050

The Shigesada-Kawasaki-Teramoto model: conditional symmetries, exact solutions and their properties

We study a simplification of the well-known Shigesada-Kawasaki-Teramoto model, which consists of two nonlinear reaction-diffusion equations with cross-diffusion. A complete set of Q-conditional (nonclassical) symmetries is derived using an algorithm adopted for the construction of conditional symmetries. The symmetries obtained are applied for finding a wide range of exact solutions, possible biological interpretation of some of which being presented. Moreover, an alternative application of the simplified model related to the polymerisation process is suggested and exact solutions are found in this case as well.


[6] 2402.19096

Structural Stability Hypothesis of Dual Unitary Quantum Chaos

Having spectral correlations that, over small enough energy scales, are described by random matrix theory is regarded as the most general defining feature of quantum chaotic systems as it applies in the many-body setting and away from any semiclassical limit. Although this property is extremely difficult to prove analytically for generic many-body systems, a rigorous proof has been achieved for dual-unitary circuits -- a special class of local quantum circuits that remain unitary upon swapping space and time. Here we consider the fate of this property when moving from dual-unitary to generic quantum circuits focussing on the \emph{spectral form factor}, i.e., the Fourier transform of the two-point correlation. We begin with a numerical survey that, in agreement with previous studies, suggests that there exists a finite region in parameter space where dual-unitary physics is stable and spectral correlations are still described by random matrix theory, although up to a maximal quasienergy scale. To explain these findings, we develop a perturbative expansion: it recovers the random matrix theory predictions, provided the terms occurring in perturbation theory obey a relatively simple set of assumptions. We then provide numerical evidence and a heuristic analytical argument supporting these assumptions.


[7] 2402.19181

Structure of Periodic Orbit Families in the Hill Restricted 4-Body Problem

The Hill Restricted 4-Body Problem (HR4BP) is a coherent time-periodic model that can be used to represent motion in the Sun-Earth-Moon (SEM) system. Periodic orbits were computed in this model to better understand the periodic orbit family structures that exist in these types of systems. First, periodic orbits in the Circular Restricted 3-Body Problem (CR3BP) representation of the Earth-Moon (EM) system were identified. A Melnikov-type function was used to identify a set of candidate points on the EM CR3BP periodic orbits to start a continuation algorithm. A pseudo-arclength continuation scheme was then used to obtain the corresponding periodic orbit families in the HR4BP when including the effect of the Sun. Bifurcation points were identified in the computed families to obtain additional orbit families.


[8] 2402.19185

Properties of Hagen-Poiseuille flows in channel networks

We derive the main properties of adaptive Hagen-Poiseuille flows in elastic microchannel networks akin to biological veins in organisms. We show that adaptive Hagen-Poiseuille flows successfully simulate key features of \textit{Physarum polycephalum} networks, replicating physiological out-of-equilibrium phenomena like peristalsis and shuttle streaming, associated with the mechanism of nutrient transport in \textit{Physarum}. A new topological steady state has been identified for asynchronous adaptation, supporting out-of-equilibrium laminar fluxes. Adaptive Hagen-Poiseuille flows show saturation effects on the fluxes in contractile veins, as observed in animal and artificial contractile veins.