A Mathematical Model of COVID-19 Using Piecewise Derivative of Fractional Order. European Journal of Pure and Applied Mathematics, Maryland, USA, v. 18, n. 2, p. 5861, 2025. DOI: 10.29020/nybg.ejpam.v18i2.5861. Disponível em: https://www.ejpam.com/index.php/ejpam/article/view/5861. Acesso em: 5 dec. 2025.