Tamberg, T. (2010). Finding a Class of 2-groups. Proceedings of the Estonian Academy of Sciences, 59(4), 370-374. https://www.airitilibrary.com/Article/Detail?DocID=17367530-201012-201012070008-201012070008-370-374
Han, M. J. (2013). A sharp observation of fundamental group [master's thesis, National Tsing Hua University]. Airiti Library. https://doi.org/10.6843/NTHU.2013.00201