Network flow problems and permutationally concave games

We examine network problems where agents have to be connected to a source in order to obtain goods, and in which costs on different arcs are a function of the flow of goods. When all cost functions are concave, the resulting game might have an empty core. We introduce a set of problems with concave functions, called the ordered quasi-symmetric congestion problems. We show that they generate permutationally concave games, a weakening of the concept of concavity, that ensures non-emptiness of the core.