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2013
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# Hildeberto Cabral

Universidade Federal de Pernambuco, Recife, Pernambuco

## Publications:

 Cabral H. E., Amorim T. A. Subharmonic Solutions of a Pendulum Under Vertical Anharmonic Oscillations of the Point of Suspension 2017, vol. 22, no. 7, pp.  782-791 Abstract We prove the existence of subharmonic solutions in the dynamics of a pendulum whose point of suspension executes a vertical anharmonic oscillation of small amplitude. Keywords: mathematical pendulum, periodic motions, subharmonic solutions Citation: Cabral H. E., Amorim T. A.,  Subharmonic Solutions of a Pendulum Under Vertical Anharmonic Oscillations of the Point of Suspension, Regular and Chaotic Dynamics, 2017, vol. 22, no. 7, pp. 782-791 DOI:10.1134/S1560354717070024
 Carvalho A., Cabral H. E. Lyapunov Orbits in the $n$-Vortex Problem on the Sphere 2015, vol. 20, no. 3, pp.  234-246 Abstract In the phase space reduced by rotation, we prove the existence of periodic orbits of the $(n + 1)$-vortex problem emanating from a relative equilibrium formed by $n$ unit vortices at the vertices of a regular polygon at a fixed latitude and an additional vortex of intensity κ at the north pole when the ideal fluid moves on the surface of a sphere. Keywords: point vortex problem, relative equilibria, periodic orbits, Lyapunov center theorem Citation: Carvalho A., Cabral H. E.,  Lyapunov Orbits in the $n$-Vortex Problem on the Sphere, Regular and Chaotic Dynamics, 2015, vol. 20, no. 3, pp. 234-246 DOI:10.1134/S156035471503003X
 Carvalho A., Cabral H. E. Lyapunov Orbits in the $n$-Vortex Problem 2014, vol. 19, no. 3, pp.  348-362 Abstract In the reduced phase space by rotation, we prove the existence of periodic orbits of the $n$-vortex problem emanating from a relative equilibrium formed by $n$ unit vortices at the vertices of a regular polygon, both in the plane and at a fixed latitude when the ideal fluid moves on the surface of a sphere. In the case of a plane we also prove the existence of such periodic orbits in the $(n+1)$-vortex problem, where an additional central vortex of intensity κ is added to the ring of the polygonal configuration. Keywords: point vortices, relative equilibria, periodic orbits, Lyapunov center theorem Citation: Carvalho A., Cabral H. E.,  Lyapunov Orbits in the $n$-Vortex Problem, Regular and Chaotic Dynamics, 2014, vol. 19, no. 3, pp. 348-362 DOI:10.1134/S156035471403006X
 Cabral H. E., Meyer K. R., Schmidt D. S. Stability and bifurcations for the $N + 1$ vortex problem on the sphere 2003, vol. 8, no. 3, pp.  259-282 Abstract The equations of motion for $N$ vortices on a sphere were derived by V.A.Bogomolov in 1977. References to related work can be found in the book by P.K.Newton. We use the equations of motion found there to discuss the stability of a ring of $N$ vortices of unit strength at the latitude $z$ together with a vortex of strength $\kappa$ at the north pole. The regions of stability are bounded by curves $\kappa = \kappa(z)$. These curves are computed explicitly for all values of $N$. When the stability of a configuration changes, for example by varying the strength of the vortex at the north pole, bifurcations to new configurations are possible. We compute the bifurcation equations explicitly for $N = 2, 3$ and $4$. For larger values of $N$ the complexity of the formal computations becomes too great and we use a numerical value for the latitude instead. We thus derive the bifurcation equations in a semi-numerical form. As expected the new configurations look very similar to those which had been found previously for the planar case. Citation: Cabral H. E., Meyer K. R., Schmidt D. S.,  Stability and bifurcations for the $N + 1$ vortex problem on the sphere , Regular and Chaotic Dynamics, 2003, vol. 8, no. 3, pp. 259-282 DOI:10.1070/RD2003v008n03ABEH000243