Mungkasi, Sudi. “Successive Approximation, Variational Iteration, and Multistage-Analytical Methods for a SEIR Model of Infectious Disease Involving Vaccination Strategy”. Communication in Biomathematical Sciences 3, no. 2 (May 10, 2021): 114-126. Accessed October 15, 2025. https://journals.itb.ac.id/index.php/cbms/article/view/14879.