Let and be natural numbers, where . At each vertex of a regular polygon with sides, either or is written. At each step, we choose consecutive vertices and change their signs. Is it possible, starting from any configuration and performing this transformation multiple times, to obtain any other configuration?
Solution
Let . If and , , then by performing the global move
we obtain the initial configuration. The same configuration can also be obtained by performing the null move. Thus, in this case, we have two different global moves that produce the same configuration, so the answer is negative.
If and is even, then the global move changes each vertex an even number of times, so it has the same effect as the null move. Therefore, in this case, the answer is negative.
Finally, if and is odd, we will show that the answer is affirmative. For this, it suffices to show that from the configuration we can obtain the configuration . Since , there exist natural numbers and such that . The global move changes each vertex, except the first one, times, and the first vertex is changed times, thus transforming into .
In conclusion, the answer is affirmative if and is odd, and negative otherwise.