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Combinatorics Difficulty 7.8 National olympiad, round 2 Prove it North Macedonia

We are given an n×nn \times n board. Rows are labeled with numbers 11 to nn downwards and columns are labeled with numbers 11 to nn from left to right. On each field we write the number x2+y2x^2 + y^2 where (x,y)(x, y) are its coordinates. We are given a figure and can initially place it on any field. In every step we can move the figure from one field to another if the other field has not already been visited and if at least one of the following conditions is satisfied:
* the numbers in those 2 fields give the same remainders when divided by nn
* those fields are point reflected with respect to the center of the board

Can all the fields be visited in case:

a) n=4n = 4

b) n=5n = 5

Solution

a) The answer is NO.

1234
1251017
2581320
310131825
417202536
1234
2121
1010
2121
1010

On the left we have the board from the problem, on the right we have the same board, but with remainders of the values from the board instead of the values themselves.
We will denote field ii for a field with number ii written on it in the right table. Let's assume that we can visit all of the fields. That means that at some point we will visit a field ii. Obviously, when using the first type of move, we can visit any other field 11 which hasn't yet been visited. Also, it is easy to notice, that for field 11, the reflection of that field is also a field 11. That means that both types of moves lead to another field 11. Also, in the same fashion we conclude that for each step, if the figure is on the field 11, then in the step after (if that wasn't the last one) and in the step before (if that wasn't the first one) should be field 11.
Now we conclude that the first visited field 11 must be the field visited in the first step. Same way we conclude that the last visited field 11 must be the field visited in the last step. But, we know that all of fields 11 are visited consecutively, in exactly 88 moves (because there are 88 fields 11), while there are exactly 1616 moves that we have to make. This leads to contradiction.

b) The answer is YES.

12345
125101726
258132029
31013182534
41720253641
52629344150
12345
120001
203304
303304
420021
514410

We can move from any field to another with the same number written on the field in the right table by using the second move.
One idea to visit all the fields is the following:
* find the 44 pairs of the fields of types field ii and field jj, such that all 88 fields are different, in each pair iji \neq j, those two fields in one pair are symmetric, and the second member of the nn-th pair has the same value on the right board as the first member of the (n+1)(n+1)-th pair. Also, we want that all the values of the right table are mentioned through members of those pairs. For example:
((2,2),(4,4)),((1,4),(5,2)),((3,5),(3,1)),((2,1),(4,5))((2,2),(4,4)),((1,4),(5,2)),((3,5),(3,1)),((2,1),(4,5))
* Now, the algorithm is: after second member of nn-th pair and before the first member of the (n+1)(n+1)-th pair visit all fields by using the first step. Of course, before first pair and after fourth pair move in similar way. Jump from the first member of the pair to the second member of the pair by using second step.
This is one of the ways to do it: We start with the field (3,3)(3,3). Then we visit all of the field 33, using the first move, in any way as long as the last visited field is (2,2)(2,2). Then, using the second move, we visit the field (4,4)(4,4). Again, using the first move we visit all fields 22 in any way as long as the last visited field is (1,4)(1,4). Using the second move we visit the field (5,2)(5,2). Then, using the first move we visit all fields 44 in any way as long as the last visited field (3,5)(3,5). In same fashion, using the second move we visit the field (4,5)(4,5) using the second move. We conclude by visiting all fields 11 in any way.

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