Primes in consecutive numbers

Is there a set of 2015 consecutive positive integers containing exactly 15 prime numbers ?

No Yes

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1 solution

Armain Labeeb
Jul 29, 2016

The answer is y e s \boxed{yes} .

For each positive integer n n let f ( n ) f(n) be the number of prime numbers among n , n + 1 , , n + 2014 n, n+1, \dots, n+2014 . I will show that f ( k ) = 15 f(k) = 15 for some positive integer number k k .

First of all we note that

  • f ( 1 ) 15 f(1) ≥ 15 since 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 \text{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47} are prime numbers.
  • f ( 2016 ! + 2 ) = 0 f(2016!+2) = 0 since for each 2 l 2016 2 ≤ l ≤ 2016 the number 2016 ! + l 2016!+l is not a prime number.

Now note that by the definition for each positive n n the difference f ( n + 1 ) f ( n ) f(n + 1) - f(n) is equal to 0, -1 or 1 \text{0, -1 or 1} . In other words, while n n increases by 1 1 , f ( n ) f(n) can change only by 1 1 .

Thus, when n changes from 1 1 to 2016 ! + 2 2016! + 2 , f ( n ) f(n) smoothly (at most by 1 1 ) changes from some number exceeding 15 15 to 0 0 . Therefore, for some integer 1 < k < 2016 ! + 2 1 < k < 2016! + 2 we have f ( k ) = 15 f(k) = 15 .

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