Eun-Kyung Cho, Ilkyoo Choi, Yiting Jiang, Ringi Kim, Boram Park,·2020-02-19·via math.CO updates on arXiv.org
This paper disproves a conjecture of Wang, Wu, Yan and Xie, and answers in negative a question in Dvorak, Pekarek and Sereni. In return, we pose five open problems.