Hakim, L., and L. Widayanti. “BOUNDEDNESS AND EXISTENCE ANALYSIS SOLUTION OF AN OPTIMAL CONTROL PROBLEMS ON MATHEMATICAL COVID-19 MODEL”. BAREKENG: Jurnal Ilmu Matematika Dan Terapan, Vol. 18, no. 2, May 2024, pp. 0797-08, doi:10.30598/barekengvol18iss2pp0797-0808.