Compute the number of positive integers that divide at least two of the integers in the set .
Solution
For a positive integer , let \operatorname{rad} n be the product of the distinct prime factors of . Observe that if , all prime factors of must divide , so \operatorname{rad} n \mid m. Therefore, if is such an integer, \operatorname{rad} n must divide at least two of the numbers in , implying that rad is either , or 5. These have , and 5 cases, respectively, for a total of 22.
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