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Consider an (n,n) matrix containing the integers from 1 to n². The matrix is “friendly” if the set of the sums of the rows is equal to the set of the sum of the columns. Find examples for n=4,5,6. Why is there no friendly matrix when n=9?

Checking for small n’s seems easy enough:

friend=function(n){
s=1
while (s>0){
A=matrix(sample(1:n^2),ncol=n)
s=sum(abs(sort(apply(A,1,sum))-
sort(apply(A,2,sum))))}
A
}

produces right answers. But the case n=6 proved itself almost too hard for brute-force handling!!! I have no time to devise a simulated-annealing code to speed up the resolution so this will have to wait till the next weekend edition of Le Monde. As to why n=9 does not enjoy a solution… (When n=2 there is no solution, as proven by the brute force exhibition of all cases.)