Find all integers that have more than divisors.
Solution
Answer: .
Clearly a number cannot have divisors greater than , besides itself. Therefore, in order to have more than divisors it must be divisible by all numbers from to and by . Denote by the integer number which equals either or . Then if , either or is coprime with . Hence, , because it must be divisible by both and . However, in this case , or , which contradicts the assumption that . All numbers can be checked by hand.
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