., Widowati, U Fitriani, Kartono ., R Heri Utomo, and Eka Triyana. “Maximum Principle for Optimal Control of COVID-19 Spread”. International Journal Of Mathematics And Computer Research 10, no. 6 (June 23, 2022): 2744-2749. Accessed May 19, 2026. https://www.ijmcr.everant.org/index.php/ijmcr/article/view/425.