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Combinatorics Difficulty 8.1 Shortlist Prove it Taiwan

Let a1,a2,,a123a_1, a_2, \dots, a_{123} be positive integers such that
* a1,a2,,a123a_1, a_2, \dots, a_{123} is a permutation of 1,2,,1231, 2, \dots, 123, and
* a1a2,a2a3,,a122a123|a_1 - a_2|, |a_2 - a_3|, \dots, |a_{122} - a_{123}| is a permutation of 1,2,,1221, 2, \dots, 122.
Prove that max(a1,a123)32\max(a_1, a_{123}) \ge 32.

Solution

Consider the following general proposition:
Let NN be a positive integer, and let a1,a2,,a2N1a_1, a_2, \dots, a_{2N-1} be positive integers satisfying the following conditions:
a1,a2,,a2N1a_1, a_2, \dots, a_{2N-1} is a permutation of 1,2,,2N11, 2, \dots, 2N-1
a1a2,a2a3,,a2N2a2N1|a_1 - a_2|, |a_2 - a_3|, \dots, |a_{2N-2} - a_{2N-1}| is a permutation of 1,2,,2N21, 2, \dots, 2N-2
Then a1+a2N1N+1a_1 + a_{2N-1} \ge N + 1, from which it follows that max(a1,a2N1)N+12\max(a_1, a_{2N-1}) \ge \lfloor \frac{N+1}{2} \rfloor. The original problem is the case N=62N = 62.

We prove the general proposition. Define the score of a number a{1,2,,2N1}a \in \{1, 2, \dots, 2N-1\} to be
s(a):=aN. s(a) := |a - N|.
By the triangle inequality we know
abaN+Nb=s(a)+s(b). |a - b| \ge |a - N| + |N - b| = s(a) + s(b).
Considering the sum a1a2+a2a3++a2N2a2N1|a_1 - a_2| + |a_2 - a_3| + \dots + |a_{2N-2} - a_{2N-1}|, we obtain
(N1)(2N1)=a1a2+a2a3++a2N2a2N12(s(a1)+s(a2)++s(a2N1))(s(a1)+s(a2N1))=2N(N1)(s(a1)+s(a2N1)). \begin{aligned} (N-1)(2N-1) &= |a_1 - a_2| + |a_2 - a_3| + \dots + |a_{2N-2} - a_{2N-1}| \\ &\le 2(s(a_1) + s(a_2) + \dots + s(a_{2N-1})) - (s(a_1) + s(a_{2N-1})) \\ &= 2N(N-1) - (s(a_1) + s(a_{2N-1})). \end{aligned}
Here the last equality holds because s(a1),s(a2),,s(a2N1)s(a_1), s(a_2), \dots, s(a_{2N-1}) is a permutation of 0,1,1,2,2,,N1,N10, 1, 1, 2, 2, \dots, N-1, N-1.

Hence, s(a1)+s(a2N1)2N(N1)(N1)(2N1)=N1s(a_1) + s(a_{2N-1}) \le 2N(N-1) - (N-1)(2N-1) = N-1. From this we obtain
(Na1)+(Na2N1)s(a1)+s(a2N1)N1, (N - a_1) + (N - a_{2N-1}) \le s(a_1) + s(a_{2N-1}) \le N - 1,
that is, a1+a2N1N+1a_1 + a_{2N-1} \ge N + 1.

Therefore max(a1,a2N1)N+12\max(a_1, a_{2N-1}) \ge \lceil \frac{N+1}{2} \rceil. For N=62N = 62, 62+12=32\lceil \frac{62+1}{2} \rceil = 32, so max(a1,a123)32\max(a_1, a_{123}) \ge 32.

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