the remaining three colours can be used on at most 6k elements. Then there remain at least four uncoloured elements. In both
cases we get a contradiction. To see that six colours are enough observe Figs. 2(a) and (b) if k = 3 and 2, respectively. To get
a colouring ofW2k+1, for k ≥ 4 it suffices to repeat k−3 the times the part from vertex x to vertex y starting with the graph
of W7 on Fig. 2(a)