Maths Olympiad Prep

Library / /37 of 105

Number theory Difficulty 5.5 AIME, harder Prove it JBMO

Problem:
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

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.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.