Resumen
Given a directed graph G, a covering is a subset B of arcs which meets all directed cuts of G. Equivalently, the contraction of the elements of B makes G strongly connected. An O(n5) primal-dual algorithm is presented by Frank (1981) for finding a minimum covering of a directed graph. For a planar graph, the dual problem is to find a minimum set of arcs whose removal makes G acyclic. The dual problem may be solved with Frank's algorithm. Further, some improvements that may be used to make such algorithm faster in practical cases are prescuted.