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Paths of fixed length in complete directed graph

Source: China Mathematical Olympiad 2016 Q6

December 17, 2015
combinatorics

Problem Statement

Let GG be a complete directed graph with 100100 vertices such that for any two vertices x,yx,y one can find a directed path from xx to yy.
a) Show that for any such GG, one can find a mm such that for any two vertices x,yx,y one can find a directed path of length mm from xx to yy (Vertices can be repeated in the path)
b) For any graph GG with the properties above, define m(G)m(G) to be smallest possible mm as defined in part a). Find the minimim value of m(G)m(G) over all such possible GG's.