Jocelyn Thiebaut: On the approximation hardness of geodetic set and its variants

Given a graph G, a geodetic set (resp. edge geodetic set) is a subset X of its vertices such that every vertex (resp. edge) of G is on a shortest path between two vertices of X. A strong geodetic set is a subset X of vertices AND a choice of a shortest path for every pair of vertices of X such that every vertex is on one of these shortest paths. The geodetic number (resp. edge geodetic number) of a graph is the minimum size of a geodetic set (resp. edge geodetic set) and the strong geodetic number is the minimum size of a strong geodetic set. In this work we prove that geodetic number and edge geodetic number are both LOG-APX-hard, even on subcubic bipartite graphs with arbitrarily high girth. Then, we show that there is no 781/780 polynomial-time approximation algorithm for strong (edge) geodetic number, even on subcubic bipartite graphs with arbitrarily high girth. We also prove that, given a subset of vertices, it is NP-hard to determine whether it is a strong geodesic set. Therefore, it is natural to study the problem of maximizing the number of covered vertices by a choice of a shortest path for every pair of a provided subset of vertices. We provide a tight 2-approximation algorithm to solve this problem. Finally, we disprove a conjecture of Iršič and Konvalinka by proving that the strong geodetic number can be computed in polynomial time in complete multipartite graphs.

Joint work with Tom Davot and Lucas Isenmann.

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