Consider the following general proposition:
Let N be a positive integer, and let a1,a2,…,a2N−1 be positive integers satisfying the following conditions:
• a1,a2,…,a2N−1 is a permutation of 1,2,…,2N−1
• ∣a1−a2∣,∣a2−a3∣,…,∣a2N−2−a2N−1∣ is a permutation of 1,2,…,2N−2
Then a1+a2N−1≥N+1, from which it follows that max(a1,a2N−1)≥⌊2N+1⌋. The original problem is the case N=62.
We prove the general proposition. Define the score of a number a∈{1,2,…,2N−1} to be
s(a):=∣a−N∣.
By the triangle inequality we know
∣a−b∣≥∣a−N∣+∣N−b∣=s(a)+s(b).
Considering the sum ∣a1−a2∣+∣a2−a3∣+⋯+∣a2N−2−a2N−1∣, we obtain
(N−1)(2N−1)=∣a1−a2∣+∣a2−a3∣+⋯+∣a2N−2−a2N−1∣≤2(s(a1)+s(a2)+⋯+s(a2N−1))−(s(a1)+s(a2N−1))=2N(N−1)−(s(a1)+s(a2N−1)).
Here the last equality holds because s(a1),s(a2),…,s(a2N−1) is a permutation of 0,1,1,2,2,…,N−1,N−1.
Hence, s(a1)+s(a2N−1)≤2N(N−1)−(N−1)(2N−1)=N−1. From this we obtain
(N−a1)+(N−a2N−1)≤s(a1)+s(a2N−1)≤N−1,
that is, a1+a2N−1≥N+1.
Therefore max(a1,a2N−1)≥⌈2N+1⌉. For N=62, ⌈262+1⌉=32, so max(a1,a123)≥32.