The Steiner ratio for the dual normed plane

A minimum Steiner tree for a given set X of points is a network interconnecting the points of X having minimal possible total length. The Steiner ratio for a metric space is the largest lower bound for the ratio of lengths between a minimum Steiner tree and a minimum spanning tree on the same set of points in the metric space. Du et al. (1993) conjectured that the Steiner ratio on a normed plane is equal to the Steiner ratio on its dual plane. In this paper we show that this conjecture is true for Ixl ~<5.