Decide whether the integers can be arranged in the cells of a matrix (where ), such that the following conditions are satisfied:
(i) In every row, the entries add up to the same sum .
(ii) In every column, the entries also add up to this sum .
(iii) For every the ten entries with add up to .
Solution
The problem essentially asks for a magic square that satisfies an additional constant-sum property along the wrap-around diagonals.
Suppose that such an arrangement of is possible. Since the sum of all entries is , we get that is an odd number. We partition the cells into four sets: Set contains the cells with and both odd; set contains the cells with odd and even ; set contains the cells with even and odd ; and set contains the remaining cells with and both even. We denote the sum of all entries in by , respectively.
* Since and contain all cells in the odd rows, we get .
* Since and contain all cells in the even columns, we get .
* Since and contain all cells with even , we get .
Adding up these three equations yields . Since in this equation the left hand side is even and the right hand side is odd, we have the desired contradiction.