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Old 2007-11-13, 15:22   #1
Kees
 
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Dec 2005

3·5·13 Posts
Default Composite checkerboard

The numbers 1,2,...,100 are written in cells of a table 10 x 10, each number is written once. By one move, we may interchange numbers in any two cells.
After how many moves may we get a table, in which the sum of numbers in every two adjacent (by side) cells is composite.

I can prove an easy upperbound, but I think it can be sharpened
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