Problem:
A positive integer is loose if it has six positive divisors and satisfies the property that any two positive divisors of satisfy . Compute the sum of all loose positive integers less than .
Solution
Solution:
Note that the condition in the problem implies that for any divisor of , if is odd then all other divisors of cannot lie in the interval . If is even, then all other divisors cannot lie in the interval .
We first find that must be of the form or for primes and . If , the only solution is when and .
Otherwise, . Since , so . Now we can do casework on .
When , we find that cannot lie in or , so we must have . All such values for work, giving solutions .
When , we find that cannot lie in or , so we must have that , so there are no solutions in this case.
When or , the only solution occurs when (since otherwise ). This gives us the solutions and .
Adding these values of gives .
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