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Combinatorics Difficulty 7.8 National Olympiad, round 2 Prove it Vietnam

Given a 10×10×1010 \times 10 \times 10 box of 10001000 unit white cubes. An and Binh play a game with this box. An selects some bands of size 1×1×101 \times 1 \times 10 such that any two chosen bands have no common points and then changes all cells on these bands to black. Binh then selects some unit cubes and asks An what color of these cells. What is the least number of cells Binh must choose in order to determine all black cells based on An's answer?

Solution

We will prove the following general statement:

Given a 2n×2n×2n2n \times 2n \times 2n box of 8n38n^3 unit white cubes. An and Binh play a game with the box. An selects some bands of size 1×1×n1 \times 1 \times n such that any two chosen bands have no common points and then changes all cells on these bands to black. Binh then selects some unit cubes and asks An what color of these cells. In this case, Binh needs to choose at least 6n26n^2 in order to determine all black cells based on An's answer.

Let SnS_n be the set of cells that Binh uses to ask An and for each chosen cell uu, let RuR_u be the union of three 1×1×n1 \times 1 \times n bands (vertical, horizontal and diagonal ones) passing through uu.

Figure 1

Since any two chosen bands have no common point, for any black cell uu there are at least two other chosen cells on RuR_u. Otherwise, Binh cannot determine exactly which band passing through uu is black. Now, we assign a tuple (a,b,c)(a,b,c) for each cell uu of the box as follows:
* a=2a=2 if the horizontal band through uu has no more in SnS_n and a=1a=1 otherwise.
* b=2b=2 if the vertical band through uu has no more in SnS_n and b=1b=1 otherwise.
* c=2c=2 if the diagonal band through uu has no more in SnS_n and c=1c=1 otherwise.

From the above observation, two of a,b,ca,b,c are 11 and the third one is at most 22. This implies that a+b+c4a+b+c \le 4.
Let TT be the sum of all assigned numbers on the box, we have
T=uSn(a+b+c)4Sn. T = \sum_{u \in S_n} (a+b+c) \le 4|S_n|.
On the other hand, there is at least one chosen cell on each 1×1×n1 \times 1 \times n band (in any direction) of the box. This implies that T23(2n)2=24n2T \ge 2 \cdot 3(2n)^2 = 24n^2.
Therefore, we have 4Sn24n24|S_n| \ge 24n^2 or Sn6n2|S_n| \ge 6n^2.

We now have the following statements:
- In the 2×2×22 \times 2 \times 2 box, Binh needs to choose at least 66 cells.
- In the 10×10×1010 \times 10 \times 10 box, Binh needs to choose at least 150150 cells.

We first show the easiest case: 2×2×22 \times 2 \times 2 box. In this case, Binh removes two opposite cells and asks for the color of the remaining ones. It is easy to check that these chosen cells work.

Figure 2

Now we can choose 150150 cells in the 10×10×1010 \times 10 \times 10 box as follows. We divide the box into 55 layers of size 10×10×210 \times 10 \times 2 and each layer into 2525 boxes of size 2×2×22 \times 2 \times 2 and label each box as in the picture below.

Figure 3

In the ith layer (i=1,2,3,4,5i=1,2,3,4,5), Binh picks the boxes labeled i and removes any two opposite cells in these boxes and asks for the color of the remaining ones. It is easy to see that if we project chosen cells into any face of the box, the shadow covers the whole face. This implies that any chosen 1×1×n1 \times 1 \times n band of An passes through one of the chosen 2×2×22 \times 2 \times 2 boxes of Binh and Binh now can easily determine whether this band is black or not.

Hence, the least number of cells that Binh must choose is 150150.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.