1. Every planar map is four colorable, I. Discharging;Appel;Ill. J. Math.,1977
2. Every planar map is four colorable, II. Reducibility;Appel;Ill. J. Math.,1977
3. Four terminal planar Delta-Wye reducibility via rooted K2,4-minors;Demasi,2015
4. Graph Theory;Diestel,2016