Find all natural numbers such that , , , and are all prime numbers.
Hong Kong competition problems, 2016
Solution
The only solution is .
Observe that
Since , , , , , and are 7 consecutive integers, one of them must be a multiple of 7. It follows that one of the numbers is equal to 7, or otherwise it cannot be a prime. Clearly, and .
If , the 5 numbers are 7, 59, 47, 349, 16843, all of which are primes.
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