Chabrowski, J., and P. M. Girāo. “Symmetric Solutions of the Neumann Problem Involving a Critical Sobolev Exponent”. Topological Methods in Nonlinear Analysis, vol. 19, no. 1, Mar. 2002, pp. 1-27, https://apcz.umk.pl/TMNA/article/view/TMNA.2002.001.