Small doubling in ordered nilpotent groups of class 2

The aim of this paper is to present a complete description of the structure of finite subsets S of torsion-free nilpotent groups of class 2 satisfying |S2|=3|S|−2. In view of results in [12], this gives a complete description of the structure of finite subsets with the above property in any torsion-free nilpotent group.