ZHANG, X., and J. LIU. “An Analytic Study on Time-Fractional Fisher Equation Using Homotopy Perturbation Method”. Walailak Journal of Science and Technology (WJST), vol. 11, no. 11, Jan. 2014, pp. 975-8, https://wjst.wu.ac.th/index.php/wjst/article/view/844.