Freely forming groups: Trying to be rare
  A simple weakly frequency dependent model for the dynamics of a population with a finite number of types is proposed, based upon an advantage of being rare. In the infinite population limit, this model gives rise to a non-smooth dynamical system that reaches its globally stable equilibrium in finite time. This dynamical system is sufficiently simple to permit an explicit solution, built piecewise from solutions of the logistic equation in continuous time. It displays an interesting tree-like structure of coalescing components. 
Michael Baake (Bielefeld), Uwe Grimm (Milton Keynes) and Harald   Jockusch (Bielefeld)