Problems related to a de Bruijn - Erd\H{o}s theorem
  De Bruijn and Erd\H{o}s proved that every noncollinear set of n points in the plane determines at least n distinct lines. We suggest a possible generalization of this theorem in the framework of metric spaces and provide partial results on related extremal combinatorial problems. 
Xiaomin Chen (Rutgers University), Va\v{s}ek Chv\'atal (Concordia   University)