A Le Monde mathematical puzzle from after the competition:
A sequence of five integers can only be modified by subtracting an integer N from two neighbours of an entry and adding 2N to the entry. Given the configuration below, what is the minimal number of steps to reach non-negative entries everywhere? Is this feasible for any configuration?
As I quickly found a solution by hand in four steps, but missed the mathematical principle behind!, I was not very enthusiastic in trying a simulated annealing version by selecting the place to change inversely proportional to its value, but I eventually tried and also obtained the same solution:
[,1] [,2] [,3] [,4] [,5]
-3 1 1 1 1
1 -1 1 1 -1
0 1 0 1 -1
-1 1 0 0 1
1 0 0 0 0
But (update!) Jean-Louis Fouley came up with one step less!
[,1] [,2] [,3] [,4] [,5]
-3 1 1 1 1
3 -2 1 1 -2
2 0 0 1 -2
1 0 0 0 0
The second part of the question is more interesting, but again without a clear mathematical lead, I could only attempt a large number of configurations and check whether all admitted “solutions”. So far none failed.
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