“A First Order Analytical Elliptic Planar Model for 3 1 Resonance Problem”. Revista Mexicana de Astronomía y Astrofísica 14, no. 1 (January 1, 1987): 641. Accessed December 9, 2025. https://astronomia.unam.mx/journals/rmxaa/article/view/1987rmxaa. 14. 641s.