Find all positive integers $n$ such that the sum of the squares of the five smallest divisors of $n$ is a square.
Problem
Source: Saint Petersburg MO 2020 Grade 10 Problem 2
Tags: number theory
08.05.2020 11:29
I think the question requires a condition that $n$ has atleast $5$ positive divisors
P.S. 5959th post
08.05.2020 21:07
You can compress above. Let $S\triangleq \sum_{1\leqslant k\leqslant 5}d_k^2$. Note if $n$ is odd, $S\equiv 5\pmod{8}$, which is not solvable; thus $n$ is even, and $d_2=2$. Now, if $3\nmid n$, then $S\equiv 2\pmod{3}$, which is not possible for a square. Consequently, $3\mid n$, $d_3=3$, and $6\mid n$ as well. Now, $4\mid n$, then $d_4=4$ and we get $30+d_5^2=x^2$, which is not solvable. Suppose $4\nmid n$. If $d_4=5$, then we get $39+d_5^2=x^2$, yielding $d_5\in\{20,8\}$. Both cases are impossible as $n$ is not divisible by $4$. Final case is $d_4=6$, which yields $50+d_5^2=x^2$, which again has no solutions.