Let Cn denote a cycle on n vertices. Clearly, a′
f2(Cn) = 2 for n even and
a′
f2(Cn) = 3 for n odd. Plane graphs whose medial graphs have proper vertexcolorings
with only 4 colors have facial 2-acyclic chromatic index 4; for example
the wheel on six vertices (see Figure 1).
Figure