Suppose that are prime numbers forming an arithmetic progression with common difference . If , prove that .
Solution
Lemma: Suppose is prime and are primes forming an A.P. with common difference . If , we claim that .
Proof: Since is prime and every is a prime , does not divide for any . By the pigeonhole principle, there exist so that . Now , and does not divide . So must divide .
Apply the Lemma to the sequences for and . Then all such 's are factors of . So .
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