Now, let an arbitrary vertex v of G be coloured with an arbitrary colour f of L(v).
We investigate whether the colouring of v can be continued to an L-list colouring of the whole
graph. G is called free k-choosable if such an L-list colouring exists for every list assignment L
(IL(v)] = k Vv E V(G)), every vertex v and every colour f E L(v).
We prove the equivalence
of the well-known conjecture of Erd6s et al. (1979): "Every planar graph is 5-choosable" with
the following conjecture: "Every planar graph is free 5-choosable".
1. Introduction