arXiv subjects

Thanos Manos

Publications and source records attributed to Thanos Manos.

14 recordsLinked to original sources

arXiv2025

Arbor-TVB: A Novel Multi-Scale Co-Simulation Framework with a Case Study on Neural-Level Seizure Generation and Whole-Brain Propagation

Computational neuroscience has traditionally focused on isolated scales, limiting understanding of brain function across multiple levels. While microscopic models capture biophysical details of neurons, macroscopic models describe large-scale network dynamics. Integrating these scales, however, remains a significant challenge. In this study, we present a novel co-simulation framework that bridges these levels by integrating the neural simulator Arbor with The Virtual Brain (TVB) platform. Arbor enables detailed simulations from single-compartment neurons to populations of such cells, while TVB models whole-brain dynamics based on anatomical features and the mean neural activity of a brain region. By linking these simulators for the first time, we provide an example of how to model and investigate the onset of seizures in specific areas and their propagation to the whole brain. This framework employs an MPI intercommunicator for real-time bidirectional interaction, translating between discrete spikes from Arbor and continuous TVB activity. Its fully modular design enables independent model selection for each scale, requiring minimal effort to translate activity across simulators. The novel Arbor-TVB co-simulator allows replacement of TVB nodes with biologically realistic neuron populations, offering insights into seizure propagation and potential intervention strategies. The integration of Arbor and TVB marks a significant advancement in multi-scale modeling, providing a comprehensive computational framework for studying neural disorders and optimizing treatments.

q-bio.NC↗

arXiv2025

On the behavior of the Generalized Alignment Index (GALI) method for dissipative systems

The behavior of the Generalized Alignment Index (GALI) method has been extensively studied and successfully applied for the detection of chaotic motion in conservative Hamiltonian systems, yet its application to non-Hamiltonian dissipative systems remains relatively unexplored. In this work, we fill this gap by investigating the GALI's ability to identify stable fixed points, stable limit cycles, chaotic (strange) and hyperchaotic attractors in dissipative systems generated by both continuous and discrete time dynamics, and compare its performance to the analysis achieved by the computation of the spectrum of Lyapunov exponents. Through a comprehensive study of three classical dissipative models, namely the 3D Lorenz system, a modified Lorenz 4D hyperchaotic system, and the 3D generalized hyperchaotic Hénon map, we examine GALI's behavior, and possible limitations, in detecting chaotic motion, as well as the presence of different types of attractors occurring in dissipative dynamical systems. We find that the GALI successfully detects chaotic motion, as well as stable fixed points, but it faces difficulties in distinctly discriminating between stable limit cycles, chaotic attractors, and hyperchaotic motion.

nlin.CD↗

arXiv2023

Networks' modulation: How different structural network properties affect the global synchronization of coupled Kuramoto oscillators

In a large variety of systems (biological, physical, social etc.), synchronization occurs when different oscillating objects tune their rhythm when they interact with each other. The different underlying network defining the connectivity properties among these objects drives the global dynamics in a complex fashion and affects the global degree of synchrony of the system. Here we study the impact of such types of different network architectures, such as Fully-Connected, Random, Regular ring lattice graph, Small-World and Scale-Free in the global dynamical activity of a system of coupled Kuramoto phase oscillators. We fix the external stimulation parameters and we measure the global degree of synchrony when different fractions of nodes receive stimulus. These nodes are chosen either randomly or based on their respective strong/weak connectivity properties (centrality, shortest path length and clustering coefficient). Our main finding is, that in Scale-Free and Random networks a sophisticated choice of nodes based on their eigenvector centrality and average shortest path length exhibits a systematic trend in achieving higher degree of synchrony. However, this trend does not occur when using the clustering coefficient as a criterion. For the other types of graphs considered, the choice of the stimulated nodes (randomly vs selectively using the aforementioned criteria) does not seem to have a noticeable effect.

nlin.AO↗

arXiv2021

Anomalous diffusion in single and coupled standard maps with extensive chaotic phase spaces

We investigate the long-term diffusion transport and chaos properties of single and coupled standard maps. We consider model parameters that are known to induce anomalous diffusion in the maps' phase spaces, as opposed to normal diffusion which is associated with Gaussian distribution properties of the kinematic variables. This type of transport originates in the presence of the so-called accelerator modes, i.e.~non-chaotic initial conditions which exhibit ballistic transport, which also affect the dynamics in their vicinity. We first systematically study the dynamics of single standard maps, investigating the impact of different ensembles of initial conditions on their behavior and asymptotic diffusion rates, as well as on the respective time-scales needed to acquire these rates. We consider sets of initial conditions in chaotic regions enclosing accelerator modes, which are not bounded by invariant tori. These types of chaotic initial conditions typically lead to normal diffusion transport. We then setup different arrangements of coupled standard maps and investigate their global diffusion properties and chaotic dynamics. Although individual maps bear accelerator modes causing anomalous transport, the global diffusion behavior of the coupled system turns out to depend on the specific configuration of the imposed coupling. Estimating the average diffusion properties for ensembles of initial conditions, as well as measuring the strength of chaos through computations of appropriate indicators, we find conditions and systems' arrangements which systematically favor the suppression of anomalous transport and long-term convergence to normal diffusion rates.

nlin.CD↗

arXiv2021

Orbit evolution in growing stellar bars: Bar-supporting orbits at the vertical ILR region

We investigate the evolution of orbital shapes at the Inner Lindblad Resonance region of a rotating three-dimensional bar, the mass of which is growing with time. We evaluate in time-dependent models, during a 5 Gyr period, the importance of orbits with initial conditions known to play a significant role in supporting peanut-like structures in autonomous systems. These orbits are the central family of periodic orbits (x1) and vertical perturbations of it, orbits of its standard three-dimensional bifurcations at the region (x1v1 and x1v2), as well as orbits in their neighbourhood. The knowledge of the regular or chaotic character of these orbits is essential as well, because it allows us to estimate their contribution to the support of a rotating bar and, more importantly, the dynamical mechanisms that make it possible. This is calculated by means of the GALI2 index. We find that orbital patterns existing in the autonomous case, persist for longer times in the more massive bar models, and even more so in a model in which the central spheroid component of our adopted galactic potential becomes rather insignificant. The peanut-supporting orbits which we find, have a regular or, in most cases, a weakly chaotic character. There are cases in which orbits starting close to unstable periodic orbits in an autonomous model behave as regular and support the bar when its mass increases with time. An approximation for the orbital dynamics of our non-autonomous models at a certain time, can be considered that of the corresponding frozen system around that time.

astro-ph.GA↗

arXiv2020

On the behavior of the Generalized Alignment Index (GALI) method for regular motion in multidimensional Hamiltonian systems

We investigate the behavior of the Generalized Alignment Index of order $k$ (GALI$_k$) for regular orbits of multidimensional Hamiltonian systems. The GALI$_k$ is an efficient chaos indicator, which asymptotically attains positive values for regular motion when $2\leq k \leq N$, with $N$ being the dimension of the torus on which the motion occurs. By considering several regular orbits in the neighborhood of two typical simple, stable periodic orbits of the Fermi-Pasta-Ulam-Tsingou (FPUT) $β$ model for various values of the system's degrees of freedom, we show that the asymptotic GALI$_k$ values decrease when the index's order $k$ increases and when the orbit's energy approaches the periodic orbit's destabilization energy where the stability island vanishes, while they increase when the considered regular orbit moves further away from the periodic one for a fixed energy. In addition, performing extensive numerical simulations we show that the index's behavior does not depend on the choice of the initial deviation vectors needed for its evaluation.

nlin.CD↗

arXiv2015

Studies of dynamical localization in a finite-dimensional model of the quantum kicked rotator

We review our recent works on the dynamical localization in the quantum kicked rotator (QKR) and the related properties of the classical kicked rotator (the standard map, SM). We introduce the Izrailev $N$-dimensional model of the QKR and analyze the localization properties of the Floquet eigenstates [{\em Phys. Rev. E} {\bf 87}, 062905 (2013)], and the statistical properties of the quasienergy spectra. We survey normal and anomalous diffusion in the SM, and the related accelerator modes [{\em Phys. Rev. E} {\bf 89}, 022905 (2014)]. We analyze the statistical properties [{\em Phys. Rev. E} {\bf 91},042904 (2015)] of the localization measure, and show that the reciprocal localization length has an almost Gaussian distribution which has a finite variance even in the limit of the infinitely dimensional model of the QKR, $N\rightarrow \infty$. This sheds new light on the relation between the QKR and the Anderson localization phenomenon in the one-dimensional tight-binding model. It explains the so far mysterious strong fluctuations in the scaling properties of the QKR. The reason is that the finite bandwidth approximation of the underlying Hamilton dynamical system in the Shepelyansky picture [{\em Phys. Rev. Lett.} {\bf 56}, 677 (1986)] does not apply rigorously. These results call for a more refined theory of the localization length in the QKR and in similar Floquet systems, where we must predict not only the mean value of the inverse of the localization length but also its (Gaussian) distribution. We also numerically analyze the related behavior of finite time Lyapunov exponents in the SM and of the $2\times2$ transfer matrix formalism.

nlin.CD↗

arXiv2014

Survey on the role of accelerator modes for anomalous diffusion: The case of the standard map

We perform an extensive and detailed analysis of the generalized diffusion processes in deterministic area preserving maps with noncompact phase space, exemplified by the standard map, with the special emphasis on understanding the anomalous diffusion arising due to the accelerator modes. The accelerator modes and their immediate neighborhood undergo ballistic transport in phase space, and also the greater vicinity of them is still much affected ("dragged") by them, giving rise to the non-Gaussian (accelerated) diffusion. The systematic approach rests upon the following applications: the GALI method to detect the regular and chaotic regions and thus to describe in detail the structure of the phase space, the description of the momentum distribution in terms of the Lévy stable distributions, the numerical calculation of the diffusion exponent and of the corresponding diffusion constant. We use this approach to analyze in detail and systematically the standard map at all values of the kick parameter $K$, up to $K=70$. All complex features of the anomalous diffusion are well understood in terms of the role of the accelerator modes, mainly of period 1 at large $K\ge 2π$, but also of higher periods (2,3,4,...) at smaller values of $K\le 2π$.

nlin.CD↗

arXiv2013

Dynamical localization in chaotic systems: spectral statistics and localization measure in the kicked rotator as a paradigm for time-dependent and time-independent systems

We study quantum kicked rotator in the classically fully chaotic regime, in the domain of the semiclassical behaviour. We use Izrailev's N-dimensional model for various N<=4000, which in the limit N-> infinity tends to the quantized kicked rotator, not only for K=5 as studied previously, but for many different values of the classical kick parameter 5<=K<=35, and also of the quantum parameter k. We describe the dynamical localization of chaotic eigenstates as a paradigm for other both time-periodic and time-independent fully chaotic or/and mixed type Hamilton systems. We generalize the localization length L and the scaling variable (L/N) to the case of anomalous classical diffusion. We study the generalized classical diffusion also in the regime where the simple minded theory of the normal diffusion fails. We greatly improve the accuracy of the numerical calculations with the following conclusions: The level spacings distribution of the eigenphases is very well described by the Brody distribution, systematically better than by other proposals, for various Brody exponents. When N->infinity and L is fixed we have always Poisson, even in fully chaotic regime. We study the eigenfunctions of the Floquet operator and characterize their localization properties using the information entropy measure describing the degree of dynamical localization of the eigenfunction. The resulting localization parameter is found to be almost equal to the Brody parameter. We show the existence of a scaling law between the localization parameter and the scaling variable L/N, now including the regimes of anomalous diffusion. The above findings are important also in time-independent Hamilton systems, like in mixed type billiards, where the Brody distribution is confirmed to a very high degree of precision for dynamically localized chaotic eigenstates.

nlin.CD↗

arXiv2013

The intermediate level statistics in dynamically localized chaotic eigenstates

We demonstrate that the energy or quasienergy level spacing distribution in dynamically localized chaotic eigenstates is excellently described by the Brody distribution, displaying the fractional power law level repulsion. This we show in two paradigmatic systems, namely for the fully chaotic eigenstates of the kicked rotator at K=7, and for the chaotic eigenstates in the mixed-type billiard system (Robnik 1983), after separating the regular and chaotic eigenstates by means of the Poincaré Husimi function, at very high energies with great statistical significance (587654 eigenstates, starting at about 1.000.000 above the ground state). This separation confirms the Berry-Robnik picture, and is performed for the first time at such high energies.

nlin.CD↗

arXiv2013

Dynamical localization in kicked rotator as a paradigm of other systems: spectral statistics and the localization measure

We study the intermediate statistics of the spectrum of quasi-energies and of the eigenfunctions in the kicked rotator, in the case when the corresponding system is fully chaotic while quantally localized. As for the eigenphases, we find clear evidence that the spectral statistics is well described by the Brody distribution, notably better than by the Izrailev's one, which has been proposed and used broadly to describe such cases. We also studied the eigenfunctions of the Floquet operator and their localization. We show the existence of a scaling law between the repulsion parameter with relative localization length, but only as a first order approximation, since another parameter plays a role. We believe and have evidence that a similar analysis applies in time-independent Hamilton systems.

nlin.CD↗

arXiv2011

Complex statistics in Hamiltonian barred galaxy models

We use probability density functions (pdfs) of sums of orbit coordinates, over time intervals of the order of one Hubble time, to distinguish weakly from strongly chaotic orbits in a barred galaxy model. We find that, in the weakly chaotic case, quasi-stationary states arise, whose pdfs are well approximated by $q$-Gaussian functions (with $1<q<3$), while strong chaos is identified by pdfs which quickly tend to Gaussians ($q=1$). Typical examples of weakly chaotic orbits are those that "stick" to islands of ordered motion. Their presence in rotating galaxy models has been investigated thoroughly in recent years due of their ability to support galaxy structures for relatively long time scales. In this paper, we demonstrate, on specific orbits of 2 and 3 degree of freedom barred galaxy models, that the proposed statistical approach can distinguish weakly from strongly chaotic motion accurately and efficiently, especially in cases where Lyapunov exponents and other local dynamic indicators appear to be inconclusive.

nlin.CD↗

arXiv2010

Scaling with system size of the Lyapunov exponents for the Hamiltonian Mean Field model

The Hamiltonian Mean Field (HMF) model is a prototype for systems with long-range interactions. It describes the motion of $N$ particles moving on a ring, coupled through an infinite-range potential. The model has a second order phase transition at the energy $U_c=3/4$ and its dynamics is exactly described by the Vlasov equation in the $N \to \infty$ limit. Its chaotic properties have been investigated in the past, but the determination of the scaling with $N$ of the Lyapunov Spectrum (LS) of the model remains a challenging open problem. We here show that the $N^{-1/3}$ scaling of the Maximal Lyapunov Exponent (MLE), found in previous numerical and analytical studies, extends to the full LS; not only, scaling is "precocious" for the LS, meaning that it becomes manifest for a much smaller number of particles than the one needed to check the scaling for the MLE. Besides that, the $N^{-1/3}$ scaling appears to be valid not only for $U>U_c$, as suggested by theoretical approaches based on a random matrix approximation, but also below a threshold energy $U_t \approx 0.2$. Using a recently proposed method (GALI) devised to rapidly check the chaotic or regular nature of an orbit, we find that $U_t$ is also the energy at which a sharp transition from {\it weak} to {\it strong} chaos is present in the phase-space of the model. Around this energy the phase of the vector order parameter of the model becomes strongly time dependent, inducing a significant untrapping of particles from a nonlinear resonance.

nlin.CD↗

arXiv2008

Explicit Construction of First Integrals with Quasi-monomial Terms from the Painlevé Series

The Painlevé and weak Painlevé conjectures have been used widely to identify new integrable nonlinear dynamical systems. For a system which passes the Painlevé test, the calculation of the integrals relies on a variety of methods which are independent from Painlevé analysis. The present paper proposes an explicit algorithm to build first integrals of a dynamical system, expressed as `quasi-polynomial' functions, from the information provided solely by the Painlevé - Laurent series solutions of a system of ODEs. Restrictions on the number and form of quasi-monomial terms appearing in a quasi-polynomial integral are obtained by an application of a theorem by Yoshida (1983). The integrals are obtained by a proper balancing of the coefficients in a quasi-polynomial function selected as initial ansatz for the integral, so that all dependence on powers of the time $τ=t-t_0$ is eliminated. Both right and left Painlevé series are useful in the method. Alternatively, the method can be used to show the non-existence of a quasi-polynomial first integral. Examples from specific dynamical systems are given.

nlin.SI↗