MMO 164 Moscow MO 1949 12 points on a circle, 4 checkers
Source:
July 20, 2019
combinatoricspointscircleColoringcombinatorial geometry
Problem Statement
There are points on a circle. Four checkers, one red, one yellow, one green and one blue sit at neighboring points. In one move any checker can be moved four points to the left or right, onto the fifth point, if it is empty. If after several moves the checkers appear again at the four original points, how might their order have changed?