1. Characterizations of metric spaces by the use of their midsets: Intervals;Berard;Fund. Math.,1971
2. Characterizations of metric spaces by the use of their midsets: One-spheres;Berard;Notices Am. Math. Soc.,1972
3. A new definition of the circle by the use of bisector;Berard;Fund. Math.,1974
4. The Geometry of Geodesics;Busemann,1955
5. Topology;Hocking,1961