Is there a set of 2015 consecutive positive integers containing exactly 15 prime numbers ?
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The answer is y e s .
For each positive integer n let f ( n ) be the number of prime numbers among n , n + 1 , … , n + 2 0 1 4 . I will show that f ( k ) = 1 5 for some positive integer number k .
First of all we note that
Now note that by the definition for each positive n the difference f ( n + 1 ) − f ( n ) is equal to 0, -1 or 1 . In other words, while n increases by 1 , f ( n ) can change only by 1 .
Thus, when n changes from 1 to 2 0 1 6 ! + 2 , f ( n ) smoothly (at most by 1 ) changes from some number exceeding 1 5 to 0 . Therefore, for some integer 1 < k < 2 0 1 6 ! + 2 we have f ( k ) = 1 5 .