crow on regular n-gon
Source: Indonesia IMO 2007 TST, Stage 2, Test 5, Problem 3
November 15, 2009
combinatorics proposedcombinatorics
Problem Statement
On each vertex of a regular n\minus{}gon there was a crow. Call this as initial configuration. At a signal, they all flew by and after a while, those crows came back to the n\minus{}gon, one crow for each vertex. Call this as final configuration. Determine all such that: there are always three crows such that the triangle they formed in the initial configuration and the triangle they formed in the final configuration are both right-angled triangle.