MathDB
Jumping frogs

Source: China TST 2003

June 29, 2006
modular arithmeticcombinatorics unsolvedcombinatorics

Problem Statement

There is a frog in every vertex of a regular 2n-gon with circumcircle(n2n \geq 2). At certain time, all frogs jump to the neighborhood vertices simultaneously (There can be more than one frog in one vertex). We call it as \textsl{a way of jump}. It turns out that there is \textsl{a way of jump} with respect to 2n-gon, such that the line connecting any two distinct vertice having frogs on it after the jump, does not pass through the circumcentre of the 2n-gon. Find all possible values of nn.