Kapanadze, D. V. “On the Uniqueness of a Solution of the Problem With Oblique Derivative for the Equation Δ n v = 0”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 6, June 2006, pp. 835–841, https://umj.imath.kiev.ua/index.php/umj/article/view/3499.