We give an O(n4) algorithm to find a minimum clique cover of a (bull, C4)-free graph, or equivalently, a minimum colouring of a (bull, 2K2)-free graph, where n is the number of vertices of the graphs.
@article{arxiv.1704.00316,
title = {Solving the clique cover problem on (bull, $C_4$)-free graphs},
author = {Kathie Cameron and Chính T. Hoàng},
journal= {arXiv preprint arXiv:1704.00316},
year = {2017}
}