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Combinatorics Difficulty 7.0 National Olympiad Prove it Taiwan

There's a convex 3n3n-polygon on the plane with a robot on each of its vertices. Each robot fires a laser beam toward another robot. On each of your move, you select a robot to rotate clockwise until its laser points at a new robot. Three robots A,BA, B and CC form a triangle, if AA's laser points at BB, BB's laser points at CC, and CC's laser points at AA. Find the minimum number of moves that can guarantee nn triangles on the plane.

Solution

Answer: 9n27n2\frac{9n^2-7n}{2} moves.

Below, EXEX denotes the expected value of the random variable XX.

1. First, for any two points AA and BB, let NABN_{AB} be the number of steps required to make AA's laser point at BB. Suppose we choose distinct AA and BB uniformly at random from all the points, then ENAB=3n22EN_{AB} = \frac{3n-2}{2} (in fact, it is easy to see that NABN_{AB} is uniformly distributed on {0,1,,3n2}\{0,1,\dots,3n-2\}.)

2. For any three points AA, BB and CC, let NABCN_{ABC} be the number of steps required to rotate the lasers on the three points so that ABCABC becomes a triangle. Suppose we choose distinct triples (A,B,C)(A,B,C) uniformly at random from all the points, then by additivity of expectation, we know ENABC=ENAB+ENBC+ENCA=3(3n2)2EN_{ABC} = EN_{AB} + EN_{BC} + EN_{CA} = \frac{3(3n-2)}{2}.

3. Denote these 3n3n points by A1A_1 through A3nA_{3n}. We use T=(T1,T2,,Tn)T = (T_1,T_2,\dots,T_n) to denote a way of partitioning all the points into nn triangles, where Ti=(Ai1,Ai2,Ai3)T_i = (A_{i1},A_{i2},A_{i3}) is an ordered triple. Let NTN_T denote the number of steps required to turn the given configuration into TT. Suppose we choose TT uniformly at random among all possible partitions, then as in point 2, by additivity, ENT=3n(3n2)2EN_T = \frac{3n(3n-2)}{2}.

4. We use S=(S1,S2,,Sn)S = (S_1,S_2,\dots,S_n) to denote a way of partitioning all the points into nn unordered sets, each of three points. For each TT, let S(T)S(T) be the corresponding unordered grouping of TT. Note that the random variable in the third point can be viewed as first choosing an SS uniformly at random among all possible SS, and then considering all permutations of SiS_i uniformly at random. Based on ENT=3n(3n2)2EN_T = \frac{3n(3n-2)}{2}, this means there exists an SS^* such that E[NTS(T)=S]3n(3n2)2E[N_T|S(T) = S^*] \le \frac{3n(3n-2)}{2}.

5. Now, note that all TT satisfying S(T)=SS(T) = S^* have the same grouping into triples of points, differing only in the clockwise or counterclockwise orientation of some of the triangles. Consider the following proposition:

Proposition 1. For a triangle, the number of steps required to turn it clockwise differs from the number of steps required to turn it counterclockwise.

If Proposition 1 holds, then it means {NTS(T)=S}\{N_T|S(T) = S^*\} contains at least n+1n+1 distinct values, so among all TT satisfying S(T)=SS(T) = S^* there must exist a TT^* such that NTE[NTS(T)=S]n23n(3n2)2n2=9n27n2N_{T^*} \le E[N_T|S(T) = S^*] - \frac{n}{2} \le \frac{3n(3n-2)}{2} - \frac{n}{2} = \frac{9n^2-7n}{2}, thereby obtaining the claimed lower bound.

6. To prove Proposition 1, consider an unordered triangle ABCABC, and suppose that the number of steps required to turn it into a clockwise triangle is the same as the number of steps required to turn it into a counterclockwise triangle. Without loss of generality, assume ABCABC is a clockwise triangle; furthermore, let n1n_1 be the number of steps required to rotate BCBC clockwise to BABA, n2n_2 be the number of steps required to rotate CACA clockwise to CBCB, and n3n_3 be the number of steps required to rotate ABAB counterclockwise to ACAC, then we have n1+n2=n3n_1 + n_2 = n_3. However, note that rotating BCBC clockwise to BABA, together with rotating CACA clockwise to CBCB, will pass through every point on the arc BACBAC exactly once, but rotating ABAB counterclockwise to ACAC will pass through every point on the arc BACBAC "except AA", which means the two operation counts differ by one, so the original assumption cannot hold.

7. Finally, we prove that this lower bound is optimal. Consider a regular 3n3n-gon, whose vertices are labeled clockwise in order as A1,A2,,A3nA_1, A_2, \dots, A_{3n}, and consider the situation where robot A1A_1 points at A2A_2, while all other robots point at A1A_1. Consider the case where, in the end, aa counterclockwise triangles are turned out; then one can compute that 3n(3n1)22n+a\frac{3n(3n-1)}{2} - 2n + a operations are required, so when a=0a = 0 we get the minimum value 3n(3n1)22n=9n27n2\frac{3n(3n-1)}{2} - 2n = \frac{9n^2-7n}{2}.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.