MathDB
Seeds within an nxn board

Source: Iberoamerican Olympiad 1990, Problem 5

May 21, 2007
analytic geometrycombinatorics proposedcombinatorics

Problem Statement

AA and BB are two opposite vertices of an n×nn \times n board. Within each small square of the board, the diagonal parallel to ABAB is drawn, so that the board is divided in 2n22n^{2} equal triangles. A coin moves from AA to BB along the grid, and for every segment of the grid that it visits, a seed is put in each triangle that contains the segment as a side. The path followed by the coin is such that no segment is visited more than once, and after the coins arrives at BB, there are exactly two seeds in each of the 2n22n^{2} triangles of the board. Determine all the values of nn for which such scenario is possible.