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Combinatorics Difficulty 7.3 National olympiad, round 2 Prove it Saudi Arabia

Consider S={(x,y,z)x,y,z{1,2,,2012}}S = \{(x, y, z) \mid x, y, z \in \{1, 2, \dots, 2012\}\} as a set of 201232012^3 points in three-dimensional space. For any segment joining two points (x1,y1,z1)(x_1, y_1, z_1) and (x2,y2,z2)(x_2, y_2, z_2) in the space, we define its distance triplet to be the ordered triple
(x1x2,y1y2,z1z2). (|x_1 - x_2|, |y_1 - y_2|, |z_1 - z_2|).
Alice wants to draw segments in such a way that

a. Each segment joins two distinct points in SS;

b. Each point in SS is an endpoint of at most one segment;

c. For any two segments, their distance triplets are different.

Find the greatest number of segments that Alice can draw.

Solutions — 2

Solution 1

We claim that Alice can draw up to K=201232K = \frac{2012^3}{2} segments. Since there are 201232012^3 points, conditions (a) and (b) guarantee that Alice can draw at most KK segments. We will prove that she can do so.

Let T={1,2,,2012}T = \{1, 2, \dots, 2012\}. We will define a bijection f:TTf: T \to T such that for all distinct i,jTi, j \in T, the inequality
f(i)if(j)j |f(i) - i| \neq |f(j) - j|
holds. We let
(f(1),f(2),,f(2012))=(2012,2011,,1510,504,1509,1508,,1008,1006,1005,,505,503,502,,1,1007). \begin{aligned} & (f(1), f(2), \dots, f(2012)) \\ = & (2012, 2011, \dots, 1510, 504, 1509, 1508, \dots, 1008, 1006, \\ & \qquad 1005, \dots, 505, 503, 502, \dots, 1, 1007). \end{aligned}
It is not hard to verify that ff satisfies the desired inequality condition.

For each point (x,y,z)S(x, y, z) \in S such that z1006z \le 1006, Alice draws a segment between it and the point (f(x),f(y),2013z)(f(x), f(y), 2013 - z). The KK segments she has drawn are easily seen to satisfy (a) and (b). To verify that they satisfy (c), suppose that two segments have the same distance triplets. Assume that the first segment has (x1,y1,z1)(x_1, y_1, z_1) where z11006z_1 \le 1006 as an endpoint, and the second segment has (x2,y2,z2)(x_2, y_2, z_2) where z21006z_2 \le 1006 as an endpoint. The distance triplets of the two segments are
(f(x1)x1,f(y1)y1,20132z1) (|f(x_1) - x_1|, |f(y_1) - y_1|, |2013 - 2z_1|)
and
(f(x2)x2,f(y2)y2,20132z2) (|f(x_2) - x_2|, |f(y_2) - y_2|, |2013 - 2z_2|)
respectively. For them to coincide, we must have that
(x1,y1,z1)=(x2,y2,z2), (x_1, y_1, z_1) = (x_2, y_2, z_2),
a contradiction.

Solution 2

We will use the following auxiliary result:

Lemma. If n0(mod4)n \equiv 0 \pmod{4}, then there exists a permutation σSn\sigma \in S_n such that
{σ(i)i:i=1,,n}={0,1,,n1}.(1) \{|\sigma(i) - i| : i = 1, \dots, n\} = \{0, 1, \dots, n-1\}. \quad (1)
Proof of Lemma. Consider the cycle defined by
σ=(1,n,2,n1,,n4,3n4+1,n4+1,3n41,,n21,n2+1,n2). \sigma = (1, n, 2, n-1, \dots, \frac{n}{4}, \frac{3n}{4}+1, \frac{n}{4}+1, \frac{3n}{4}-1, \dots, \frac{n}{2}-1, \frac{n}{2}+1, \frac{n}{2}).
If n=4kn = 4k, then we have
σ(1)1=4k1σ(2k+1)(2k+1)=1 |\sigma(1) - 1| = 4k - 1 \qquad |\sigma(2k + 1) - (2k + 1)| = 1
σ(2)2=4k3σ(2k+2)(2k+2)=3 |\sigma(2) - 2| = 4k - 3 \qquad |\sigma(2k + 2) - (2k + 2)| = 3
σ(k)k=2k+1σ(3k1)(3k1)=2k3 |\sigma(k) - k| = 2k + 1 \qquad |\sigma(3k - 1) - (3k - 1)| = 2k - 3
σ(k+1)(k+1)=2k2σ(3k)3k=0 |\sigma(k + 1) - (k + 1)| = 2k - 2 \qquad |\sigma(3k) - 3k| = 0
σ(k+2)(k+2)=2k4σ(3k+1)(3k+1)=2k+2 |\sigma(k + 2) - (k + 2)| = 2k - 4 \qquad |\sigma(3k + 1) - (3k + 1)| = 2k + 2
σ(2k1)(2k1)=2σ(4k1)(4k1)=4k4 |\sigma(2k - 1) - (2k - 1)| = 2 \qquad |\sigma(4k - 1) - (4k - 1)| = 4k - 4
σ(2k)2k=2k1σ(4k)4k=4k2. |\sigma(2k) - 2k| = 2k - 1 \qquad |\sigma(4k) - 4k| = 4k - 2.
Remark. Such a permutation exists if and only if
n0(mod4) or n1(mod4). n \equiv 0 \pmod{4} \text{ or } n \equiv 1 \pmod{4}.
To see the condition is necessary, notice that those nn distinct differences must be, in some order, the numbers 0,1,,n10, 1, \dots, n-1, as 0σ(k)kn1,k=1,,n0 \le |\sigma(k) - k| \le n-1, k = 1, \dots, n. We must have
n(n1)2=k=1nσ(k)kk=1n(σ(k)k)0(mod2). \frac{n(n-1)}{2} = \sum_{k=1}^{n} |\sigma(k) - k| \equiv \sum_{k=1}^{n} (\sigma(k) - k) \equiv 0 \pmod{2}.
In our problem, clearly 20120(mod4)2012 \equiv 0 \pmod{4}. We connect the point (x,y,z)(x, y, z) with (σ(x),σ(y),2013z)(\sigma(x), \sigma(y), 2013 - z), where z1006z \le 1006, x,y2012x, y \le 2012, and σS2012\sigma \in S_{2012} is the permutation in Lemma. The "distance" of a such segment is (xσ(x),yσ(y),20132z)(|x - \sigma(x)|, |y - \sigma(y)|, 2013 - 2z). Moreover, for two pairs (x1,y1,z1),(x2,y2,z2),z1,z21006(x_1, y_1, z_1), (x_2, y_2, z_2), z_1, z_2 \le 1006, we have
{x1σ(x1)=x2σ(x2)y1σ(y1)=y2σ(y2)20132z1=20132z2 \left\{ \begin{array}{l} |x_1 - \sigma(x_1)| = |x_2 - \sigma(x_2)| \\ |y_1 - \sigma(y_1)| = |y_2 - \sigma(y_2)| \\ 2013 - 2z_1 = 2013 - 2z_2 \end{array} \right.
so all the "distances" are different and every point lies in some segment. It follows that we have 201232\frac{2012^3}{2} segments.

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