Determine all positive integers with at least factors such that is the sum the squares of its smallest factors.
Solution
1. Let be the smallest four positive divisors of . We know that .
2. Given that is the sum of the squares of its four smallest factors, we have:
Substituting , we get:
Therefore:
3. If were odd, then would all be odd, making even, which is a contradiction. Hence, must be even, implying .
4. Substituting into the equation, we get:
5. Since is even, must be odd. This means one of and is odd while the other is even.
Case 1: If is even, say :
- If , then must be 3, and:
However, does not divide 30, so is not a solution.
- If , since must be a divisor (as is a divisor), and:
Since :
Therefore:
We need to check if 130 satisfies the problem's conditions. The divisors of 130 are 1, 2, 5, 10, 13, 26, 65, and 130. The smallest four divisors are 1, 2, 5, and 10. We check:
Thus, satisfies the conditions.
Case 2: If is even, and there were some such that :
- Then would be a divisor between and , which is a contradiction. So .
- Since yields , which is absurd, and . Hence, if we say :
From , we get , so . However, this yields:
This does not satisfy the conditions since 4 does not divide 70.
Thus, the only that satisfies the problem's conditions is .
The final answer is .