On the Families of Periodic Orbits of the Sitnikov Problem. - Jaume Llibre; Rafael Ortega (SIAM J. Appl. Dyn. Syst., 2008)

Some Rigorous Results on the 1: 1 Resonance of the Spin-Orbit Problem. - Mauricio Misquero; Rafael Ortega (SIAM J. Appl. Dyn. Syst., 2020)