Example 4 Given . Let be a sequence of numbers from , and it includes all permutations of that do not end with 1, i.e., if is a permutation of and , then there exist , such that
. Find the minimum value of the number of terms in such a sequence.
(1991 Shanghai Competition Problem)
Solution
Since includes all permutations of where the second number is 1, there exists
such that is a permutation of , , and is a sequence composed of elements from , and includes all permutations of any two elements from . It is easy to see that must have at least 5 terms, i.e., .
Clearly, adding any term to the sequence would not satisfy the required property, so .
On the other hand, it is easy to verify that the sequence includes all permutations of that do not end with 1. Therefore, the minimum value of the number of terms is 11.
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