Given a graph, the eccentricity of a vertex v is defined as the greatest distance from v to any other vertex. A center (also: centroid) of a graph is a vertex with minimal eccentricity. A graph can have an arbitrary number of centers. However, Jordan (1869) has proved that for trees, there are only two possibilities:
- The tree has precisely one center (centered trees).
- The tree has precisely two centers (bicentered trees). In this case, the two centers are adjacent.
A proof of this fact is given, for example, by Knuth.
- (Knuth 1997), p. 387 and p. 589
- Jordan, Camille (1869). "Sur les assemblages de lignes". Journal für die reine und angewandte Mathematik (in French). 70 (2): 185–190.
- Knuth, Donald E. (1997). The Art of Computer Programming, Volume 1: Fundamental Algorithms (3rd ed.). Addison-Wesley Professional. ISBN 0-201-89683-4.
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