Portnyagin, D. V. “Boundedness of Weak Solutions of a Nondiagonal Singular Parabolic System of Three Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 8, Aug. 2006, pp. 1084–1096, https://umj.imath.kiev.ua/index.php/umj/article/view/3515.