MathDB
Connected Monochromatic k-subset

Source: Turkey TST 2002 - P6

April 6, 2013
combinatorics proposedcombinatorics

Problem Statement

Consider 2n+12n+1 points in space, no four of which are coplanar where n>1n>1. Each line segment connecting any two of these points is either colored red, white or blue. A subset MM of these points is called a connected monochromatic subset, if for each a,bMa,b \in M, there are points a=x0,x1,,xl=ba=x_0,x_1, \dots, x_l = b that belong to MM such that the line segments x0x1,x1x2,,xl1x1x_0x_1, x_1x_2, \dots, x_{l-1}x_1 are all have the same color. No matter how the points are colored, if there always exists a connected monochromatic kk-subset, find the largest value of kk. (l>1l > 1)