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Selection and Training Session

Belarus counting and probability

Problem

We call a coloring of an table () 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 () for which there exists a good coloring of table.
Solution
Answer: all such that divides both of them.
Final answer
All pairs where both dimensions are multiples of six.

Techniques

Coloring schemes, extremal argumentsInvariants / monovariants