Rachma, Andina Aulia, and Rizky Rosjanuardi. “Students’ Obstacles in Learning Sequence and Series Using Onto-Semiotic Approach”. Jurnal Pendidikan Matematika, vol. 15, no. 2, June 2021, pp. 115-32, https://jpm.ejournal.unsri.ac.id/index.php/jpm/article/view/231.