In this paper, we consider only 2-connected plane graphs since any non-2-
connected plane graph contains a face whose boundary is not a cycle.
Clearly, for any 2-connected plane graph facial 1-acyclic edge-coloring coincides
with facial 2-acyclic edge-coloring (since face-adjacent edges have different
colors).
Lemma 1. Let G be a 2-connected plane graph. Then 2 ≤ a′
f2(G) ≤ 4.
Moreover, these bounds are tight.