Farber, Michael. “Zeros of Closed 1-Forms, Homoclinic Orbits and Lusternik-Schnirelman Theory”. Topological Methods in Nonlinear Analysis 19, no. 1 (March 1, 2002): 123–152. Accessed February 12, 2026. https://apcz.umk.pl/TMNA/article/view/TMNA.2002.008.