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Einstein-Maxwell-dilaton neutral black holes in strong magnetic fields: Topological charge, shadows, and lensing

Physical Review D 2022年第6期105卷 064070-064070页

作者： Haroldo C. D. Lima Junior Jian-Zhi Yang Luís C. B. Crispino Pedro V. P. Cunha Carlos A. R. Herdeiro Departamento de Matemática da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA) Campus de Santiago 3810-183 Aveiro Portugal Programa de Pós-Graduação em Física Universidade Federal do Pará 66075-110 Belém Pará Brasil

The light rings (LRs) topological charge (TC) of a spacetime measures the number of stable LRs minus the number of unstable LRs. It is invariant under smooth spacetime deformations obeying fixed boundary conditions. Asymptotically flat equilibrium black holes (BHs) have, generically, TC=−1. In Einstein-Maxwell theory, however, the Schwarzschild-Melvin BH—describing a neutral BH immersed in a strong magnetic field—has TC=0. This allows the existence of BHs without LRs and produces remarkable phenomenological features, like panoramic shadows. Here we investigate the generalized Schwarzschild-Melvin solution in Einstein-Maxwell-dilaton theory, scanning the effect of the dilaton coupling a. We find that the TC changes discontinuously from TC=0 to TC=−1 precisely at the Kaluza-Klein value a=3, when the (empty) Melvin solution corresponds to a twisted Kaluza-Klein reduction of five-dimensional flat spacetime, i.e., the dilaton coupling a induces a topological transition in the TC. We relate this qualitative change to the Melvin asymptotics for different a. We also study the shadows and lensing of the generalized Schwarzschild-Melvin solution for different values of a, relating them to the TC.

关键词： Classical black holes

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The H-join of arbitrary families of graphs

arXiv

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arXiv 2021年

作者： Cardoso, Domingos M. Gomes, Helena Pinheiro, Sofia J. Center for Research and Development in Mathematics and Applications Department of Mathematics University of Aveiro Campus de Santiago Aveiro3810-193 Portugal Center for Research and Development in Mathematics and Applications Escola Superior de Ed-ucação Instituto Politécnico de Viseu Viseu Portugal

The H-join of a family of graphs G = {G1, . . ., Gp}, also called the generalized composition, H[G1, . . ., Gp], where all graphs are undirected, simple and finite, is the graph obtained by replacing each vertex i of H by Gi and adding to the edges of all graphs in G the edges of the join Gi ∨ Gj, for every edge ij of H. Some well known graph operations are particular cases of the H-join of a family of graphs G as it is the case of the lexicographic product (also called composition) of two graphs H and G, H[G]. During long time the known expressions for the determination of the entire spectrum of the H-join in terms of the spectra of its components and an associated matrix were limited to families of regular graphs. In this work, we extend such a determination, as well as the determination of the characteristic polynomial, to families of arbitrary graphs. From the obtained results, the eigenvectors of the adjacency matrix of the H-join can also be determined in terms of the adjacency matrices of the components and an associated *** Codes 05C50, 05C76 © 2021, CC BY-NC-ND.

关键词： Graphic methods

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Absorption and unbounded superradiance in a static regular black hole spacetime

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Physical Review D 2024年第6期109卷 064053-064053页

作者： Marco A. A. de Paula Luiz C. S. Leite Sam R. Dolan Luís C. B. Crispino Programa de Pós-Graduação em Física Universidade Federal do Pará 66075-110 Belém Pará Brazil Consortium for Fundamental Physics School of Mathematics and Statistics University of Sheffield Hicks Building Hounsfield Road Sheffield S3 7RH United Kingdom Campus Altamira Instituto Federal do Pará 68377-630 Altamira Pará Brazil Departamento de Matemática da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA) Campus de Santiago 3810-183 Aveiro Portugal

Regular black holes (RBHs)—geometries free from curvature singularities—arise naturally in theories of nonlinear electrodynamics. Here we study the absorption and superradiant amplification of a monochromatic planar wave in a charged, massive scalar field impinging on the electrically charged Ayón-Beato-García (ABG) RBH. Comparisons are drawn with absorption and superradiance for the Reissner-Nordström (RN) black hole in linear electrodynamics. We find that, in a certain parameter regime, the ABG absorption cross section is negative, due to superradiance, and moreover it is unbounded from below as the momentum of the wave approaches zero; this phenomenon of “unbounded superradiance” is absent in the RN case. We show how the parameter space can be divided into regions, using the bounded/unbounded and absorption/amplification boundaries. After introducing a high-frequency approximation based on particle trajectories, we calculate the absorption cross section numerically, via the partial-wave expansion, as a function of wave frequency, and we present a gallery of results. The cross section of the ABG RBH is found to be larger (smaller) than in the RN case when the field charge has the same (opposite) sign as the black hole charge. We show that it is possible to find “mimics”: situations in which the cross sections of both black holes are very similar. We conclude with a discussion of unbounded superradiance and superradiant instabilities.

关键词： General relativity Gravitation

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GENERALIZED q-FOCK SPACES AND STRUCTURAL IDENTITIES

arXiv

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arXiv 2023年

作者： Alpay, Daniel Cerejeiras, Paula Kaehler, Uwe Schneider, Baruch Faculty of Mathematics Physics and Computation Schmid College of Science and Technology Chapman University One University Drive OrangeCA92866 United States Center for research and development in mathematics and applications Department of mathematics University of Aveiro Campus Universitário de Santiago Aveiro3810-193 Portugal University of Ostrava Department of Mathematics 30.dubna 22 Ostrava70200 Czech Republic

Using q-calculus we study a family of reproducing kernel Hilbert spaces which interpolate between the Hardy space and the Fock space. We give characterizations of these spaces in terms of classical operators such as integration and backward-shift operators, and their q-calculus counterparts. Furthermore, these new spaces allow us to study intertwining operators between classic backward-shift operators and the q-Jackson *** Codes 30H20, 26A33 © 2023, CC BY-NC-ND.

关键词： Algebra

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Generating gravitational waveform libraries of exotic compact binaries with deep learning

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Physical Review D 2024年第12期109卷 124059-124059页

作者： Felipe F. Freitas Carlos A. R. Herdeiro António P. Morais António Onofre Roman Pasechnik Eugen Radu Nicolas Sanchis-Gual Rui Santos Departamento de Física Centre for Research and Development in Mathematics and Applications (CIDMA) Campus de Santiago 3810-183 Aveiro Portugal Departamento de Matemática Centro de Física das Universidades do Minho e do Porto (CF-UM-UP) Department of Astronomy and Theoretical Physics Departamento de Astronomía y Astrofísica ISEL—Instituto Superior de Engenharia de Lisboa Centro de Física Teórica e Computacional Faculdade de Ciências

Current gravitational wave (GW) detections rely on the existence of libraries of theoretical waveforms. Consequently, finding new physics with GWs requires libraries of nonstandard models, which are computationally demanding. We discuss how deep learning frameworks can be used to generate new waveforms “learned” from a simulation dataset obtained, say, from numerical relativity simulations. Concretely, we use the WaveGAN architecture of a generative adversarial network (GAN). As a proof of concept we provide this neural network (NN) with a sample of (>500) waveforms from the collisions of exotic compact objects (Proca stars), obtained from numerical relativity simulations. Dividing the sample into a training and a validation set, we show that after a sufficiently large number of training epochs the NN can produce from 12% to 25% of the synthetic waveforms with an overlapping match of at least 95% with the ones from the validation set. We also demonstrate that a NN can be used to predict the overlapping match score, with 90% accuracy, of new synthetic samples. These are encouraging results for using GANs for data augmentation and interpolation in the context of GWs, to cover the full parameter space of, say, exotic compact binaries, without the need for intensive numerical relativity simulations.

关键词： Gravitation Gravitational wave detection Gravitational wave sources Gravitational waves

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Aspects of regular and singular electromagnetic-generalized-quasitopological-gravities black holes in 2+1 dimensions

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Physical Review D 2024年第2期109卷 024027-024027页

作者： Jeferson de Oliveira R. D. B. Fontana A. B. Pavan Instituto de Física Universidade Federal de Mato Grosso Cuiabá Mato Grosso 78060-900 Brazil Universidade Federal do Rio Grande do Sul Campus Tramandaí-RS Estrada Tramandaí-Osório Rio Grande do Sul 95590-000 Brazil and Departamento de Matemática Universidade de Aveiro and Center for Research and Development in Mathematics and Applications (CIDMA) Campus Santiago 3810-183 Aveiro Portugal Instituto de Física e Química Universidade Federal de Itajubá Itajubá Minas Gerais 37500-903 Brazil

We investigate quasitopological black holes in (2+1) dimensions in the context of electromagnetic-generalized-quasitopological gravities (EM-GQT). For three different families of geometries of quasitopological nature, we study the causal structure and their response to a probe scalar field. To linear order, we verify that the scalar field evolves stably, decaying in different towers of quasinormal modes. The studied black holes are either charged geometries (regular and singular) or a regular Bañados-Teitelboim-Zanelli (BTZ)-like black hole, both coming from the EM-GQT theory characterized by nonminimal coupling parameters between gravity and a background scalar field. We calculate the quasinormal modes applying different numerical methods with convergent results between them. The oscillations demonstrate a very peculiar structure for charged black holes: in the intermediate and near extremal cases, a particular scaling arises, similar to that of the rotating BTZ geometry, with the modes being proportional to the distance between horizons. For the single horizon black hole solution, we identify the presence of different quasinormal families by analyzing the features of that spectrum. In all three considered geometries, no instabilities were found.

关键词： Alternative gravity theories Fluids & classical fields in curved spacetime Gravity in dimensions other than four Spacetime topology & causal structure

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Marketing and religion - Spiritual commitment and church engagement of a Portuguese Parish: A principal component analysis

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AIP Conference Proceedings 2024年第1期3094卷

作者： Jéssica Campos Manuel José Fonseca Helena Sofia Rodrigues Business School Instituto Politécnico de Viana do Castelo Viana do Castelo Portugal UNIAG – Applied Management Research Unit Porto Portugal CIDMA - Center for Research & Development in Mathematics and Applications University of Aveiro Aveiro Portugal

Marketing is an area that has always been updated over time, considering it fundamental to adapt to the changes and evolution of the market and consumers. Consequently, the enlargement of marketing to new areas has been a growing reality. Based on this reality and within the scope of Religious Marketing, a study was carried out for the Parish of Poiares, in Portugal. The general objective was to characterize the profile of the parishioners, with the intention of subsequently defining strategies and implementing religious marketing tactics in the context of the parish of Poiares. To this end, an empirical work of qualitative nature was carried out, through the application of a questionnaire survey to a non-probabilistic convenience sample of 120 individuals. This empirical work had as its main objectives to understand the level of spirituality/involvement of the parishioners and the factors that register their greater or lesser satisfaction.

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Evaluation of seaport equipment performance using multi-directional efficiency analysis for sustainable logistics operations: A case from Portugal

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research in Transportation Business & Management 2025年 62卷

作者： Syed M. Kazmi Eugénio M. Rocha Ângela F. Brochado Department of Mathematics (DMat) University of Aveiro 3810-193 Aveiro Portugal Center for Research and Development in Mathematics and Applications (CIDMA) University of Aveiro 3810-193 Aveiro Portugal Department of Economics Management Industrial Engineering and Tourism (DEGEIT) University of Aveiro 3810-193 Aveiro Portugal

Seaports are critical nodes in global supply chains, making it essential to analyze equipment efficiency to optimize their operations, reduce costs and operational times, while also acknowledging environmental concerns to meet current stricter sustainability goals. This study evaluates the performance of cranes and loading arms at the Portuguese Port of Sines, employing Multi-Directional Efficiency Analysis (MEA) to address limitations in traditional efficiency measurement methods like Data Envelopment Analysis (DEA). MEA uniquely accounts for both desirable (cargo weight) and undesirable (CO 2 emissions) output variables, alongside operational input variables (energy consumption and duration). The analysis reveals significant efficiency differences across equipment categories, uncovering unexpected insights for equipment managers. It reveals consistently underperforming units, such as the GRUA LIEBHERR 40 T crane, and exemplary performers, including the QC09 crane and Braço de Carga 9. Notably, this study bridges a research gap by introducing MEA to seaport logistics, offering a comprehensive evaluation framework that integrates environmental and operational metrics. Key findings highlight actionable insights for decision-makers, e.g., prioritizing equipment upgrades or defining stricter maintenance schedules. These findings underscore MEA's potential as a decision-support tool for enhancing resource utilization, reducing costs, and fostering sustainable operations in the maritime logistics sector. In addition, future research directions are explored in the manuscript's last section.

关键词： Multi-directional efficiency analysis Data-driven decision-making Equipment efficiency analysis Sustainable operations Seaport

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Optimization of Cognitive Radio System Using Enhanced Firefly Algorithm

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Intelligent Automation & Soft Computing 2023年第9期37卷 3159-3177页

作者： Nitin Mittal Rohit Salgotra Abhishek Sharma Sandeep Kaur SSAskar Mohamed Abouhawwash University Center for Research and Development Chandigarh UniversityMohaliPunjab140413India Faculty of Physics and Applied Computer Science AGH University of Science&TechnologyKrakowPoland Faculty of Information Technology Middle East UniversityAmman11813Jordan Department of Computer Engineering and Applications GLA UniversityMathura281406India Department of Computer Engineering&Technology Guru Nanak Dev UniversityAmritsarPunjab143005India Department of Statistics and Operations Research College of ScienceKing Saud UniversityP.O.Box 2455Riyadh11451Saudi Arabia Department of Computational Mathematics Science and EngineeringCollege of EngineeringMichigan State UniversityEast LansingMI48824USA Department of Mathematics Faculty of ScienceMansoura UniversityMansoura35516Egypt

The optimization of cognitive radio(CR)system using an enhanced firefly algorithm(EFA)is presented in this *** Firefly algorithm(FA)is a nature-inspired algorithm based on the unique light-flashing behavior of *** has already proved its competence in various optimization prob-lems,but it suffers from slow convergence *** improve the convergence performance of FA,a new variant named EFA is *** effectiveness of EFA as a good optimizer is demonstrated by optimizing benchmark functions,and simulation results show its superior performance compared to biogeography-based optimization(BBO),bat algorithm,artificial bee colony,and *** an application of this algorithm to real-world problems,EFA is also applied to optimize the CR *** is a revolutionary technique that uses a dynamic spectrum allocation strategy to solve the spectrum scarcity ***,it requires optimization to meet specific performance *** results obtained by EFA in CR system optimization are compared with results in the literature of BBO,simulated annealing,and genetic *** results further prove that the proposed algorithm is highly efficient and provides superior results.

关键词： Firefly algorithm cognitive radio bit error rate genetic algorithm simulated annealing biogeography-based optimization

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Epicyclic frequencies for a generic ultracompact object

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Physical Review D 2022年第6期106卷 064054-064054页

作者： Jorge F. M. Delgado Departamento de Matemática Universidade de Aveiro Center for Research and Development in Mathematics and Applications-CIDMA Campus de Santiago Aveiro 3810-183 Portugal Centro de Astrofísica e Gravitação-CENTRA Departamento de Física Instituto Superior Técnico-IST Universidade de Lisboa-UL Av. Rovisco Pais 1 Lisboa 1049-001 Portugal

Recently, it has been shown that the radial stability of a light ring (LR) in a spacetime generated by a stationary, axisymmetric, asymptotically flat object with a Z2 symmetry determines the possibility and radial stability of timelike circular orbits (TCOs) around the LR. In this paper, we generalize this result by also considering the vertical (angular) stability of the orbits through the study of the radial and vertical epicyclic frequencies. We show that the vertical stability of the LR only determines the vertical stability of the TCOs around it. A relation between the sum of the squared epicyclic frequencies and the Ricci tensor is also provided. With such relation, we show that objects with radially and vertically unstable LRs (TCOs) violate the null (strong) energy condition.

关键词： General relativity Gravitation

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