Maths Olympiad Prep

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Combinatorics Difficulty 5.6 AIME, harder Find the answer

Example 4 Given S={1,2,3,4}S=\{1,2,3,4\}. Let a1,a2,,aka_{1}, a_{2}, \cdots, a_{k} be a sequence of numbers from SS, and it includes all permutations of (1,2,3,4)(1,2,3,4) that do not end with 1, i.e., if (b1,b2,b3,b4)\left(b_{1}, b_{2}, b_{3}, b_{4}\right) is a permutation of (1,2,3,4)(1,2,3,4) and b41b_{4} \neq 1, then there exist 1i1<i2<i3<i4k1 \leqslant i_{1}<i_{2}<i_{3}<i_{4} \leqslant k, such that
(ai1,ai2,ai3,ai4)=(b1,b2,b3,b4)\left(a_{i_{1}}, a_{i_{2}}, a_{i_{3}}, a_{i_{4}}\right)=\left(b_{1}, b_{2}, b_{3}, b_{4}\right). Find the minimum value of the number of terms kk in such a sequence.
(1991 Shanghai Competition Problem)

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Since a1,a2,,aka_{1}, a_{2}, \cdots, a_{k} includes all permutations of (1,2,3,4)(1,2,3,4) where the second number is 1, there exists
1i1<i2<i3<i4<i5<<imk, 1 \leqslant i_{1}<i_{2}<i_{3}<i_{4}<i_{5}<\cdots<i_{m} \leqslant k,

such that (ai1,ai2,ai3)\left(a_{i_{1}}, a_{i_{2}}, a_{i_{3}}\right) is a permutation of (2,3,4)(2,3,4), ai4=1a_{i_{4}}=1, and ai5,,aima_{i_{5}}, \cdots, a_{i_{m}} is a sequence composed of elements from {2,3,4}\{2,3,4\}, and includes all permutations of any two elements from {2,3,4}\{2,3,4\}. It is easy to see that ai5,,aima_{i_{5}}, \cdots, a_{i_{m}} must have at least 5 terms, i.e., m9m \geqslant 9.

Clearly, adding any term to the sequence ai1,,aima_{i_{1}}, \cdots, a_{i_{m}} would not satisfy the required property, so k11k \geqslant 11.

On the other hand, it is easy to verify that the sequence 1,3,2,4,1,2,3,1,4,3,21,3,2,4,1,2,3,1,4,3,2 includes all permutations of (1,2,3,4)(1,2,3,4) that do not end with 1. Therefore, the minimum value of the number of terms kk is 11.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.