Number theoryDifficulty 5.2AIME, harderFind the answer
Find the smallest positive integer n such that n2′s2222>100 factorials ((⋯((100!)!)!⋯)!)!
A number or a short expression. Spacing and $ signs are ignored.
Solution
Note that 2222>1002. We claim that a>b2⟹2a>(b!)2, for b>2. This is because 2a>b2b⟺a>2blog2(b) and log2(b)<b2/2 for b>2. Then since bb>b ! this bound works. Then m2′s(222⋯2)>m−4 factorials ((((100!)!)!)!…)2 for all m≥4 by induction. So n=104 works. The lower bound follows from the fact that n!>2n for n>3, and since 100>222, we have 100 factorials (((100!)!)!)!…)>1002′s22⋯2100>10322⋯2
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