For every positive integer n , does there always exist n consecutive positive integers such that none of them can be written in the form p k , where p is a prime number and k is a positive integer greater than 1?
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Consider the following set S of n consecutive integers S = { ( ( n + 1 ) ! ) 2 + j , j = 2 , 3 , … , n + 1 } . Any element from the set S may be factorized as follows ( ( n + 1 ) ! ) 2 + j = j ( 1 + c j ) , for some integer c . Clearly, GCD ( j , 1 + c j ) = 1 . Hence, it can not be expressed as p k for any prime p and any integer k > 1 . ■
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Let p 1 , p 2 , … , p 2 n be distinct prime numbers. To show that there exists n consecutive integers that are not perfect powers of any prime, we only need to show that the i th number in the sequence is divisible by p 2 i − 1 p 2 i , because the extra prime factor makes it so that the number cannot possibly be a power of one prime.
Thus, we must find a positive integer x such that x + i ≡ 0 ( m o d p 2 i − 1 p 2 i ) for i = 1 , 2 , … , n . By the Chinese Remainder Theorem, there exists a solution x to the system. This choice of x guarantees that all of x + 1 , x + 2 , … , x + n are not the power of any prime, so we are done. ■
Addendum: This problem is from the 1989 IMO, #5 .