Number theoryDifficulty 5.5AIME, harderProve itJBMO
Problem: Show that there exist infinitely many positive integers n such that n2+n+14n+2n+1 is an integer.
Solution
Solution: Let f(n)=n2+n+1. Note that f(n2)=n4+n2+1=(n2+n+1)(n2−n+1) This means that f(n)∣f(n2) for every positive integer n. By induction on k, one can easily see that f(n)∣f(n2k) for every positive integers n and k. Note that the required condition is equivalent to f(n)∣f(2n). From the discussion above, if there exists a positive integer n so that 2n can be written as n2k, for some positive integer k, then f(n)∣f(2n). If we choose n=22m and k=2m−m for some positive integer m, then 2n=n2k and since there are infinitely many positive integers of the form n=22m, we have the desired result.
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