Martínez-Alfaro, José, et al. “Singular Levels and Topological Invariants of Morse--Bott Foliations on Non Orientable Surfaces”. Topological Methods in Nonlinear Analysis, vol. 51, no. 1, Jan. 2018, pp. 183-1, https://apcz.umk.pl/TMNA/article/view/TMNA.2017.051.