Before revision:  We also discuss the hardness of the problem , achieving a better lower bound than the previous one [ IPL 2019 ] . We believe our simplified method of analysis can be helpful to deal with similar problems on general undirected graphs .
After revision:  Then we prove that it is not possible to improve the algorithm significantly by showing that for every constant c the problem is W [ IPL 2019 ] . We believe our simplified method of analysis can be helpful to deal with similar problems on general undirected graphs .