CONO-SUR 1991 Problem 4
A game consists in coins (blacks or whites) arrenged in the following position (see picture 1). If you choose coin on the border of the square, this coin and it's neighbours change their color. If you choose the coin at the centre, it doesn't change it's color, but the other coins do. Here is an example of white coins, and the changes of their colors, choosing the coin said: (see picture 2). Is it possible, starting with white coins, to have black coins?.
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