Wang, J., Y. Zhou, and M. Medveď. “Existence and Stability of Fractional Differential Equations With Hadamard Derivative”. Topological Methods in Nonlinear Analysis, vol. 41, no. 1, Apr. 2013, pp. 113-3, https://apcz.umk.pl/TMNA/article/view/TMNA.2013.005.