Let be a positive integer. Elfie the Elf lives in a three dimensional space . She starts at the origin: . In each turn she can teleport into any point in which lies at the distance from her current location. However, teleportation is a complicated procedure. Elfie starts off normal but she turns strange with her first teleportation. Next time she teleports she becomes normal again, then strange again... etc.
For which can Elfie travel to any given point in and be normal when she gets there?
Solution
Answer: there are no such .
We colour all the points in white and black: The point is colored white if and black if .
After the first move Elfie is at a point where . Thus,
Now, if is even then is white. Thus, in that case Elfie only jumps between white points.
On the other hand, if is odd, then is certainly black. And one can easily see that Elfie alternates between black and white squares after each move. But since Elfie is normal after even number of moves, and is then on a white point, she can never reach any black point being normal. Thus, there no such that Elfie can travel to any given point and be normal when she gets there.
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