6.11 Validity of Algorithm 6.13 In this problem you are
asked to prove the validity of Algorithm 6.13 for the single center of a
tree. Using the notation of Section 6.5.4 and the symbol x* to
denote the (yet unknown) absolute center of the undirected tree network G:
and, therefore, that x* must lie on the path associated with m(es) and must be at the halfway point between es and et.