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The stickiness effect is a fundamental feature of quasi-integrable Hamiltonian systems. We propose the use of an entropy-based measure of the recurrence plots (RP), namely, the entropy of the distribution of the recurrence times (estimated from the RP), to characterize the dynamics of a typical quasi-integrable Hamiltonian system with coexisting regular and chaotic regions. We show that the recurrence time entropy (RTE) is positively correlated to the largest Lyapunov exponent, with a high correlation coefficient. We obtain a multi-modal distribution of the finite-time RTE and find that each mode corresponds to the motion around islands of different hierarchical levels., 16 pages, 7 figures

Area-preserving nontwist maps are used to describe a broad range of physical systems. In those systems, the violation of the twist condition leads to nontwist characteristic phenomena, such as reconnection-collision sequences and shearless invariant curves that act as transport barriers in the phase space. Although reported in numerical investigations, the shearless bifurcation, i.e., the emergence scenario of multiple shearless curves, is not well understood. In this work, we derive an area-preserving map as a local approximation of a particle transport model for confined plasmas. Multiple shearless curves are found in this area-preserving map, with the same shearless bifurcation scenario numerically observed in the original model. Due to its symmetry properties and simple functional form, this map is proposed as a model to study shearless bifurcations., 16 pages, 10 figures

In this work, we demonstrate numerically that two-frequency excitation is an effective method to produce chaotification over very large regions of the parameter space for the Duffing oscillator with single- and double-well potentials. It is also shown that chaos is robust in the last case. Robust chaos is characterized by the existence of a single chaotic attractor which is not altered by changes in the system parameters. It is generally required for practical applications of chaos to prevent the effects of fabrication tolerances, external influences, and aging that can destroy chaos. After showing that very large and continuous regions in the parameter space develop a chaotic dynamics under two-frequency excitation for the double-well Duffing oscillator, we demonstrate that chaos is robust over these regions. The proof is based upon the observation of the monotonic changes in the statistical properties of the chaotic attractor when the system parameters are varied and by its uniqueness, demonstrated by changing the initial conditions. The effects of a second frequency in the single-well Duffing oscillator is also investigated. While a quite significant chaotification is observed, chaos is generally not robust in this case.

Osciladores de fase são sistemas dinâmicos dissipativos com aporte externo de energia, e matematicamente caracterizados pela presença de ciclos-limite, ao longo dos quais a dinâmica pode ser descrita por um ângulo de fase. No presente artigo, consideramos fenômenos de sincronização num sistema de muitos osciladores de fase acoplados. Adotamos uma descrição qualitativa, baseada num modelo paradigmático proposto por Y. Kuramoto em 1975. Discutimos, à luz dessa descrição, algumas aplicações em sistemas físicos (metrônomos e pedestres numa ponte pênsil) e biológicos (neurônios e vagalumes piscando). Phase oscillators are dissipative dynamic systems with external energy input, and mathematically characterized by the presence of limit cycles, along which the dynamics can be described by a phase angle. In the present article, we consider synchronization phenomena in a system of many phase-coupled oscillators. We adopt a qualitative description, based on a paradigmatic model proposed by Y. Kuramoto in 1975. We discuss, in the light of this description, some applications in physical systems (metronomes and pedestrians on a suspension bridge) and biological (neurons and fireflies blinking).

The influence of external harmonic forcing in nonlinear chaotic systems is a subject of active investigation, chiefly in low-dimensional dynamical systems, but fewer results are available for high-dimensional systems like plasmas. In this paper, we consider a theoretical model for a weakly ionized plasma, in which the following effects are taken into account: (i) ambipolar diffusion; (ii) the inflow of plasma particles through ionization processes; (iii) the outflow of plasma particles due to recombination. The spatiotemporal patterns of the resulting nonlinear system, as revealed by numerical integration of the reaction-diffusion partial differential equations, can be partially or totally suppressed through the action of acoustic waves forming stationary patterns in the plasma container. These suppression effects are quantitatively investigated by a number of numerical diagnostics like the average Lyapunov exponents and spatial correlation function. Suppression of spatiotemporal chaos can be achieved through adequate choices of amplitude and frequency of the applied waves.

In this work, we study the unpredictability of seasonal infectious diseases considering a SEIRS model with seasonal forcing. To investigate the dynamical behaviour, we compute bifurcation diagrams type hysteresis and their respective Lyapunov exponents. Our results from bifurcations and the largest Lyapunov exponent show bistable dynamics for all the parameters of the model. Choosing the inverse of latent period as control parameter, over 70% of the interval comprises the coexistence of periodic and chaotic attractors, bistable dynamics. Despite the competition between these attractors, the chaotic ones are preferred. The bistability occurs in two wide regions. One of these regions is limited by periodic attractors, while periodic and chaotic attractors bound the other. As the boundary of the second bistable region is composed of periodic and chaotic attractors, it is possible to interpret these critical points as tipping points. In other words, depending on the latent period, a periodic attractor (predictability) can evolve to a chaotic attractor (unpredictability). Therefore, we show that unpredictability is associated with bistable dynamics preferably chaotic, and, furthermore, there is a tipping point associated with unpredictable dynamics.

Introdução: Durante a pandemia de COVID-19 tivemos um aumento do consumo de hemocomponentes com redução do número de doações voluntárias, prejudicando a gestão de hemocomponentes. Pacientes em programas de hemotransfusão crônica ou com aloimunização necessitam de concentrados de hemácias com fenótipo idênticos para os sistemas Rh e antígeno K foram impactados pela menor disponibilidade de hemocomponentes seguindo essas premissas. Objetivo: Comparar a frequência dos antígenos E, e, C, c, K entre doadores e receptores de sangue em banco de sangue privado na cidade do Rio de Janeiro durante a pandemia de COVID-19, avaliar se o município possui perfil similar entre essas 2 populações e seu impacto para gestão de hemocomponentes. Materiais e métodos: Foram analisados resultados de identificação de antígenos E, e, C, c e K de doadores voluntários e de receptores com indicação de transfusão com protocolo de fenotipagem para sistema RH e K no período de janeiro 2020 até junho 2021.Os doadores com tipagem O e A foram 100% fenotipados para os antígenos em questão. Os demais doadores foram fenotipados de acordo com a demanda do serviço de hemoterapia. Os receptores foram fenotipados conforme a indicação de aplicação do protocolo de fenotipagem. Número de indivíduos testados: Doadores 17.159 e Receptores 3.292. Resultados: Entre os doadores avaliados: 59% eram do sexo masculino, encontramos o seguinte perfil antigênico antígeno D=88%, antígeno E=25%, antígeno e=98%, antígeno C=60%, antígeno c=83%, antígeno K=6%. Importante ressaltar que dentre os doadores D negativos o percentual de antígeno c e antígeno e negativos era de 0%. Entre receptores avaliados: 45% eram do sexo masculino, encontramos o seguinte perfil antigênico antígeno D=85%, antígeno E=23%, antígeno e=98%, antígeno C=64%, antígeno c=83%, antígeno K=6%. Discussão: Investigamos o perfil fenotípico de doadores comparativo ao de receptores com base no município do Rio tomando como base o período de pandemia de COVID-19. Avaliamos sexo, naturalidade e fenótipos de doadores e receptores. Realizamos um estudo preliminar com população de doadores no Brasil em 2018, sem comparação com receptores, e encontramos aproximadamente o mesmo resultado percentual de antigenemia deste estudo. Percebemos que a antigenemia dos receptores segue a mesma proporcionalidade que a dos doadores. Conclusão: Este estudo sugere que a população da cidade do Rio de Janeiro durante a pandemia de COVID-19 possui uma frequência de antígenos do sistema Rh e Kell semelhante entre doadores × receptores, e ao mesmo tempo semelhante ao nosso estudo realizado pré pandemia apenas com doadores da região sudeste e nordeste. Podemos então até o momento supor que a metrópole Rio de janeiro possui população representativa do perfil antigênico brasileiro, com potencial para autossutentabilidade caso a cultura de doação de repetição fosse ampliada na população, reduzindo a dificuldade de atendimento principalmente para pacientes D negativos com antígenos raros como c ou e negativos. Estudos comparativos de todas as regiões do país podem auxiliar a compreensão mais fidedigna do perfil populacional a auxiliar nas estratégias de captação e retenção de doadores.

Oscillatory activities in the brain, detected by electroencephalograms, have identified synchronization patterns. These synchronized activities in neurons are related to cognitive processes. Additionally, experimental research studies on neuronal rhythms have shown synchronous oscillations in brain disorders. Mathematical modeling of networks has been used to mimic these neuronal synchronizations. Actually, networks with scale-free properties were identified in some regions of the cortex. In this work, to investigate these brain synchronizations, we focus on neuronal synchronization in a network with coupled scale-free networks. The networks are connected according to a topological organization in the structural cortical regions of the human brain. The neuronal dynamic is given by the Rulkov model, which is a two-dimensional iterated map. The Rulkov neuron can generate quiescence, tonic spiking, and bursting. Depending on the parameters, we identify synchronous behavior among the neurons in the clustered networks. In this work, we aim to suppress the neuronal burst synchronization by the application of an external perturbation as a function of the mean-field of membrane potential. We found that the method we used to suppress synchronization presents better results when compared to the time-delayed feedback method when applied to the same model of the neuronal network.

Barriers have been identified in magnetically confined plasmas reducing the particle transport and improving the confinement. One of them, the primary shearless barriers are associated to extrema of non-monotonic plasma profiles. Previously, we identified these barriers in a model described by a map that allows the integration of charged particles motion in drift waves for a long time scale. In this work, we show how the existence of these robust barriers depends on the fluctuation amplitude and on the electric shear. Moreover, we also find control parameter intervals for which these primary barriers onset and break-up are recurrent. Another noticeable feature, in these transitions, is the appearance of a layer of particle trajectory stickiness after the shearless barrier break-up or before its onset. Besides the mentioned primary barriers, we also observe sequences of secondary shearless barriers, not reported before, created and destroyed by a sequence of bifurcations as the main control parameters, the fluctuation amplitude and electric shear, are varied. Furthermore, in these bifurcations, we also find hitherto unknown double and triple secondary shearless barriers which constitute a noticeable obstacle to the chaotic transport., 12 pages, 28 figures. This article has been accepted by the Journal Physics of Plasmas. It can be found at: https://doi.org/10.1063/5.0056428

A fundamental model of tumor growth in the presence of cytotoxic chemotherapeutic agents is formulated. The model allows to study the role of the Norton-Simon hypothesis in the context of dose-dense chemotherapy. Dose-dense protocols aim at reducing the period between courses of chemotherapy from three weeks to two weeks, in order to avoid tumor regrowth between cycles. We address the conditions under which these protocols might be more or less beneficial in comparison to less dense settings, depending on the sensitivity of the tumor cells to the cytotoxic drugs. The effects of varying other parameters of the protocol, as for example the duration of each continuous drug infusion, are also inspected. We believe that the present model might serve as a foundation for the development of more sophisticated models for cancer chemotherapy.

Intrinsically nonlinear coupled systems present different oscillating components that exchange energy among themselves. We present a new approach to deal with such energy exchanges and to investigate how it depends on the system control parameters. The method consists in writing the total energy of the system, and properly identifying the energy terms for each component and, especially, their coupling. To illustrate the proposed approach, we work with the bi-dimensional spring pendulum, which is a paradigm to study nonlinear coupled systems, and is used as a model for several systems. For the spring pendulum, we identify three energy components, resembling the spring and pendulum like motions, and the coupling between them. With these analytical expressions, we analyze the energy exchange for individual trajectories, and we also obtain global characteristics of the spring pendulum energy distribution by calculating spatial and time average energy components for a great number of trajectories (periodic, quasi-periodic and chaotic) throughout the phase space. Considering an energy term due to the nonlinear coupling, we identify regions in the parameter space that correspond to strong and weak coupling. The presented procedure can be applied to nonlinear coupled systems to reveal how the coupling mediates internal energy exchanges, and how the energy distribution varies according to the system parameters., Submitted for publication

The human cerebral cortex can be separated into cortical areas forming a clustered network structure. We build two different clustered networks, where one network is based on a healthy brain and the other according to a brain affected by a neurodegenerative process. Each cortical area has a subnetwork with small-world properties. We verify that both networks exhibit rich-club organisation and phase synchronisation. Due to the fact that neuronal synchronisation can be related to brain diseases, we consider the delayed feedback control as a method to suppress synchronous behaviours. In this work, it is presented that depending on the feedback parameters, intensity and time delay, phase synchronisation in both networks can be suppressed. Therefore, one of our main results is to show that delayed feedback control can be used to suppress undesired synchronous behaviours not only in the healthy brain, but also in the brain marked by neurodegenerative processes.