Steven Roman. A problem ? multi-colorings of graphs. Discrete Mathematics, 22(1):41-47, 1978. [doi]
@article{Roman78,
title = {A problem ? multi-colorings of graphs},
author = {Steven Roman},
year = {1978},
doi = {10.1016/0012-365X(78)90045-6},
url = {http://dx.doi.org/10.1016/0012-365X(78)90045-6},
researchr = {https://researchr.org/publication/Roman78},
cites = {0},
citedby = {0},
journal = {Discrete Mathematics},
volume = {22},
number = {1},
pages = {41-47},
}