A Mathematical Model of COVID-19 Using Piecewise Derivative of Fractional Order. Eur. J. Pure Appl. Math. [Internet]. 2025 May 1 [cited 2025 Jul. 11];18(2):5861. Available from: https://www.ejpam.com/index.php/ejpam/article/view/5861