A Step Beyond Kemperman's Structure Theorem

A classical result of Kemperman gives a complete recursive description of the structure of those subsets $A$ and $B$ of an abelian group that fail to satisfy the triangle inequality, i.e., $|A+B|<|A|+|B|$. In this paper, we achieve the complete description in the case when equality holds: $|A+B|=|A|+|B|$.