Maths Olympiad Prep

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Combinatorics Difficulty 5.2 AIME, harder Find the answer

You want to arrange the numbers 1,2,3,,251,2,3, \ldots, 25 in a sequence with the following property: if nn is divisible by mm, then the nnth number is divisible by the mm th number. How many such sequences are there?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the rearranged numbers be a1,,a25a_{1}, \ldots, a_{25}. The number of pairs (n,m)(n, m) with nmn \mid m must equal the number of pairs with anama_{n} \mid a_{m}, but since each pair of the former type is also of the latter type, the converse must be true as well. Thus, nmn \mid m if and only if anama_{n} \mid a_{m}. Now for each n=1,2,,6n=1,2, \ldots, 6, the number of values divisible by nn uniquely determines nn, so n=ann=a_{n}. Similarly, 7,8 must either be kept fixed by the rearrangement or interchanged, because they are the only values that divide exactly 2 other numbers in the sequence; since 7 is prime and 8 is not, we conclude they are kept fixed. Then we can easily check by induction that n=ann=a_{n} for all larger composite numbers n25n \leq 25 (by using m=amm=a_{m} for all proper factors mm of nn ) and n=11n=11 (because it is the only prime that divides exactly 1 other number). So we have only the primes n=13,17,19,23n=13,17,19,23 left to rearrange, and it is easily seen that these can be permuted arbitrarily, leaving 4 ! possible orderings altogether.

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