Morrison, G., and A. Taheri. “Topology of Twists, Extremising Twist Paths and Multiple Solutions to the Nonlinear System in Variation $\mathscr{L}[u] = \nabla \mathscr{P}$”. Topological Methods in Nonlinear Analysis, vol. 54, no. 2, Nov. 2019, pp. 833-62, https://apcz.umk.pl/TMNA/article/view/TMNA.2019.076.