This paper disproves a conjecture in Wang et al. (Graphs Comb. 31:1779–1787, 2015) and answers in the negative a question in Dvořák et al. (Electron J Comb:P26, 2019). In return, we pose five open problems.
Supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. NRF-2020R1I1A1A0105858711). Supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (No. NRF-2018R1D1A1B07043049), and also by the Hankuk University of Foreign Studies Research Fund. Supported by the National Research Foundation of Korea grant funded by the Korea government (No. NRF-2021R1C1C1010763), and also by INHA UNIVERSITY Research Grant. Supported by the National Research Foundation of Korea grant funded by the Korea government (No. NRF-2018R1C1B6003577). Supported by NSFC 11971438, ZJNSF LD19A010001 and 111 project of the Ministry of Education of China.
