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Enigmas & puzzles

By Ian Stewart   August 2006

Desperately daubing dominoes

Sanguimenta was sitting at the kitchen table with a box of dominoes and four tubes of paint—red, yellow, green, and blue.

“Not often I see you relaxing,” said her sister Nonsequita.

“Relaxing? I’m not relaxing! I’m doing some research on the four-colour theorem!”

“What’s that?”

“Any map on a flat sheet of paper can be coloured, using at most four colours, so that adjacent countries have different colours,” said Sanguimenta. “The countries can be any shape you like. It was first proposed in 1852 but remained unproved until 1976. It’s easy to see that four…

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