To finish the proof of Theorem 3, observe that the above mentioned 5-degeneracy of GX
+ induces a partial 6-colouring ϕ
of the vertices of GX in which all vertices except of e1 are coloured. This colouring can be extended to the whole graph GX
by putting ϕ(e1) = ϕ(α3). This is possible because all neighbours of e1 (in GX
+) are adjacent to α3.