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Combinatorics Difficulty 8.8 Shortlist Prove it Baltic Way

Let nn be a positive integer. Elfie the Elf lives in a three dimensional space Z3\mathbb{Z}^3. She starts at the origin: (0,0,0)(0,0,0). In each turn she can teleport into any point in Z3\mathbb{Z}^3 which lies at the distance n\sqrt{n} 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 nn can Elfie travel to any given point in Z3\mathbb{Z}^3 and be normal when she gets there?

Solution

Answer: there are no such nn.
We colour all the points in Z3\mathbb{Z}^3 white and black: The point (x,y,z)(x, y, z) is colored white if x+y+z20x+y+z \equiv_2 0 and black if x+y+z21x + y + z \equiv_2 1.
After the first move Elfie is at a point (a,b,c)(a, b, c) where a2+b2+c2=na^2 + b^2 + c^2 = n. Thus, a+b+c2na + b + c \equiv_2 n
Now, if nn is even then (a,b,c)(a, b, c) is white. Thus, in that case Elfie only jumps between white points.
On the other hand, if nn is odd, then (a,b,c)(a, b, c) 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 nn such that Elfie can travel to any given point and be normal when she gets there.

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