Compute the sum of all positive integers such that and does not divide .
Solution
We claim that if , then if and only if both and are prime. If both and are prime, then assume . By Fermat Little Theorem, . However, since is prime, , so , a contradiction. If is composite, then is even and is at most , so !, done. If is composite but is prime, then !, so . The prime numbers between 50 and 100 are . If one of these is , then the only numbers that make prime are 53,83 , and 89 , making one of 52,82 , and 88 . These sum to 222.
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