José Luis Castro Quilantán., José Luis del Río-Correa., ., and Marco Antonio Rosales Medina. “Alternative Proof of Bertrand’s Theorem Using a Phase Space Approach”. Revista Mexicana De Física, vol. 42, no. 5, Jan. 1995, pp. 867-7, https://rmf.smf.mx/ojs/index.php/rmf/article/view/2622.