Problem:
Let be a permutation of the numbers from through . What is the maximum possible number of integers with such that divides ?
Solution
Solution:
Answer:
Since any proper divisor of must be less than or equal to , none of the numbers greater than can divide any other number less than or equal to . Since there are at most values of for which , this means that there can be at most values of for which divides .
On the other hand, there exists a permutation for which divides for exactly values of , namely the permutation:
Formally, for each odd number , we construct the sequence , where is the largest integer such that . We then concatenate all of these sequences to form a permutation of the numbers through (note that no number occurs in more than one sequence). It follows that if , then will equal , and therefore will divide for all values of satisfying .
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