Torné, O. “A One Dimensional Problem Related to the Symmetry of Minimisers for the Sobolev Trace Constant in a Ball”. Topological Methods in Nonlinear Analysis, vol. 38, no. 2, Apr. 2011, pp. 363-72, https://apcz.umk.pl/TMNA/article/view/TMNA.2011.037.