Consecutive Numbers pt. 2

For every positive integer n , n, does there always exist n n consecutive positive integers such that none of them can be written in the form p k , p^k, where p p is a prime number and k k is a positive integer greater than 1?

Yes No

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2 solutions

Steven Yuan
Jul 20, 2017

Let p 1 , p 2 , , p 2 n p_1, p_2, \dots, p_{2n} be distinct prime numbers. To show that there exists n n consecutive integers that are not perfect powers of any prime, we only need to show that the i i th number in the sequence is divisible by p 2 i 1 p 2 i , p_{2i - 1}p_{2i}, 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 x such that x + i 0 ( m o d p 2 i 1 p 2 i ) x + i \equiv 0 \! \pmod{p_{2i - 1}p_{2i}} for i = 1 , 2 , , n . i = 1, 2, \dots, n. By the Chinese Remainder Theorem, there exists a solution x x to the system. This choice of x x guarantees that all of x + 1 , x + 2 , , x + n x + 1, x + 2, \dots, x + n are not the power of any prime, so we are done. \blacksquare

Addendum: This problem is from the 1989 IMO, #5 .

Abhishek Sinha
Jul 27, 2017

Consider the following set S S of n n consecutive integers S = { ( ( n + 1 ) ! ) 2 + j , j = 2 , 3 , , n + 1 } . S= \{((n+1)!)^2+j, j=2,3, \ldots, n+1\}. Any element from the set S S may be factorized as follows ( ( n + 1 ) ! ) 2 + j = j ( 1 + c j ) , ((n+1)!)^2+j=j(1+cj), for some integer c c . Clearly, GCD ( j , 1 + c j ) = 1 \textrm{GCD}(j, 1+cj)=1 . Hence, it can not be expressed as p k p^k for any prime p p and any integer k > 1. k> 1. \blacksquare

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