Maths Olympiad Prep

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Number theory Difficulty 5.0 AIME Find the answer

Find the smallest positive integer nn for which 1!2!(n1)!>n!21!2!\cdots(n-1)!>n!^{2}

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

Solution

Dividing both sides by n!2n!^{2}, we obtain 1!2!(n3)!(n2)!(n1)![n(n1)!][n(n1)(n2)!]>11!2!(n3)!n2(n1)>11!2!(n3)!>n2(n1)\begin{aligned} \frac{1!2!\ldots(n-3)!(n-2)!(n-1)!}{[n(n-1)!][n(n-1)(n-2)!]} & >1 \\ \frac{1!2!\ldots(n-3)!}{n^{2}(n-1)} & >1 \\ 1!2!\ldots(n-3)! & >n^{2}(n-1) \end{aligned} Factorials are small at first, so we can rule out some small cases: when n=6n=6, the left hand side is 1!2!3!=121!2!3!=12, which is much smaller than 6256^{2} \cdot 5. (Similar calculations show that n=1n=1 through n=5n=5 do not work. either.) Setting n=7n=7, the left-hand side is 288 , which is still smaller than 7267^{2} \cdot 6. However, n=8n=8 gives 34560>44834560>448, so 8 is the smallest integer for which the inequality holds.

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