Solution:
We first prove that for any odd n≥3 there are n×n tables that are not "good". Consider an arbitrary n×n table and denote by Pi, i=1,2,…,n, the product of the numbers in the i-th row at the second last step. Then P1P3=P2P4=⋯=Pn−1P1=PnP2=1 and since n is odd, it follows that P1=P2=⋯=Pn. This argument shows that the row products in the initial table must be equal. Therefore any table which has not this property is not "good".
We now consider a table of order n=2km, where m is an odd number and k≥1. After the first two steps the number in the position (i,j) becomes equal to the product of the numbers in the positions (i−2,j), (i,j−2), (i,j+2) and (i+2,j). Therefore the resulting table after every even step can be obtained by applying the operation on the following four tables of order 2k−1m:
- the table of all (i,j) with i≡j≡0(mod2);
- the table of all (i,j) with i≡0(mod2), j≡1(mod2);
- the table of all (i,j) with i≡1(mod2), j≡0(mod2);
- the table of all (i,j) with i≡j≡1(mod2).
Now it follows by induction that the number n=2km has the required property if and only if the number 2k−1m does. It is also easy to see that every table of order 2 is "good".
Therefore the required n are n=2k, where k is a positive integer.