Maths Olympiad Prep

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Combinatorics Difficulty 5.5 AIME, harder Prove it Belarus

We call a coloring of an m×nm \times n table (m,n5m, n \ge 5) in three colors a *good coloring* if the following two conditions are satisfied:

1) Each cell has the same number of neighboring cells of two other colors;
2) Each corner cell has no neighboring cells of its color.

Find all pairs (m,n)(m, n) (m,n5m, n \ge 5) for which there exists a good coloring of m×nm \times n table.

Solution

Answer: all (m,n)(m, n) such that 66 divides both of them.

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