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Number theory Difficulty 5.2 AIME, harder Find the answer

Find the smallest positive integer nn such that 2222n2s>((((100!)!)!)!)!100 factorials \underbrace{2^{2^{2^{2}}}}_{n 2^{\prime} s}>\underbrace{((\cdots((100!)!)!\cdots)!)!}_{100 \text { factorials }}

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

Solution

Note that 2222>10022^{2^{2^{2}}}>100^{2}. We claim that a>b22a>(b!)2a>b^{2} \Longrightarrow 2^{a}>(b!)^{2}, for b>2b>2. This is because 2a>b2ba>2blog2(b)2^{a}>b^{2 b} \Longleftrightarrow a>2 b \log _{2}(b) and log2(b)<b2/2\log _{2}(b)<b^{2} / 2 for b>2b>2. Then since bb>bb^{b}>b ! this bound works. Then (2222)m2s>((((100!)!)!)!)2m4 factorials \underbrace{\left(2^{2^{2 \cdots 2}}\right)}_{m 2^{\prime} \mathrm{s}}>\underbrace{((((100!)!)!)!\ldots)^{2}}_{m-4 \text { factorials }} for all m4m \geq 4 by induction. So n=104n=104 works. The lower bound follows from the fact that n!>2nn!>2^{n} for n>3n>3, and since 100>222100>2^{2^{2}}, we have (((100!)!)!)!)100 factorials >2221001002s>222103\underbrace{(((100!)!)!)!\ldots)}_{100 \text { factorials }}>\underbrace{2^{2 \cdots^{2^{100}}}}_{1002^{\prime} \mathrm{s}}>\underbrace{2^{2} \cdots^{2}}_{103}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.