KalitaE. A. “Exactness of the Cordes Condition of the Hölder Property for the Gradient of Nondivergent Elliptic Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 47, no. 2, Feb. 1995, pp. 292–294, https://umj.imath.kiev.ua/index.php/umj/article/view/5415.