Salinas-Hernández, E., G. Ares de Parga, S. Domínguez-Hernández, and R. Muñoz-Vega. “Approximate Frequencies of the Pendulum for Large Angles”. Revista Mexicana De Física E, vol. 63, no. 1 Jan-Jun, Jan. 2017, pp. 6-11, https://rmf.smf.mx/ojs/index.php/rmf-e/article/view/4762.