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Number theory Difficulty 5.9 AIME, harder Prove it

Show that there exist infinitely many positive integers nn such that

4n+2n+1n2+n+1 \frac{4^{n}+2^{n}+1}{n^{2}+n+1}

is an integer.

Solution

Let f(n)=n2+n+1f(n)=n^{2}+n+1. Note that

f(n2)=n4+n2+1=(n2+n+1)(n2n+1) f\left(n^{2}\right)=n^{4}+n^{2}+1=\left(n^{2}+n+1\right)\left(n^{2}-n+1\right)

This means that f(n)f(n2)f(n) \mid f\left(n^{2}\right) for every positive integer nn. By induction on kk, one can easily see that f(n)f(n2k)f(n) \mid f\left(n^{2^{k}}\right) for every positive integers nn and kk. Note that the required condition is equivalent to f(n)f(2n)f(n) \mid f\left(2^{n}\right). From the discussion above, if there exists a positive integer nn so that 2n2^{n} can be written as n2kn^{2^{k}}, for some positive integer kk, then f(n)f(2n)f(n) \mid f\left(2^{n}\right). If we choose n=22mn=2^{2^{m}} and k=2mmk=2^{m}-m for some positive integer mm, then 2n=n2k2^{n}=n^{2^{k}} and since there are infinitely many positive integers of the form n=22mn=2^{2^{m}}, we have the desired result.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.