For k = 1, let Fk be the class of graphs that contains k vertices meeting all its cycles. The minor-obstruction set for Fk is the set obs(Fk) containing all minor-minimal graphs that do not belong to Fk. We denote by Yk the set of all outerplanar graphs in obs(Fk). I
Rue, J., Thilikos, D., Stavropoulos, K. Outerplanar obstructions for the vertex feedback set. "European journal of combinatorics", 2 Juliol 2012, vol. 33, núm. 5, p. 948-968.