Let be a positive integer such that is composite (not a prime) and divides , where is the Euler's totient function of and is the sum of the positive divisors of . Prove that has at least three distinct prime factors.
Solution
If for some prime , then . This implies , and hence . Therefore, must be squarefree.
If for some distinct primes and , then
This is a multiple of if and only if . We now prove that has no solution in distinct integers using Vieta's jumping.
Suppose on the contrary that the divisibility is solvable. Let be a solution such that and is the smallest possible. Let for some positive integer . Rewrite this equation as
This is a quadratic equation in . Let be another solution to this equation. Since , is an integer. Since , is positive. Also, we have
This shows is another pair of solution to such that . As , by the minimality of , we must have . But then the divisibility becomes , i.e. . This is impossible since . Thus, there is no solution.
It follows that must have at least 3 distinct prime divisors.