Jezierski, J., E. Keppelmann, and W. Marzantowicz. “Wecken Property for Periodic Points on the Klein Bottle”. Topological Methods in Nonlinear Analysis, vol. 33, no. 1, Mar. 2009, pp. 51-64, https://apcz.umk.pl/TMNA/article/view/TMNA.2009.005.