Find the smallest positive integer n for which 1!2!⋯(n−1)!>n!2
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Dividing both sides by n!2, we obtain [n(n−1)!][n(n−1)(n−2)!]1!2!…(n−3)!(n−2)!(n−1)!n2(n−1)1!2!…(n−3)!1!2!…(n−3)!>1>1>n2(n−1) Factorials are small at first, so we can rule out some small cases: when n=6, the left hand side is 1!2!3!=12, which is much smaller than 62⋅5. (Similar calculations show that n=1 through n=5 do not work. either.) Setting n=7, the left-hand side is 288 , which is still smaller than 72⋅6. However, n=8 gives 34560>448, so 8 is the smallest integer for which the inequality holds.
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