Determine if there is a set of 2011 positive integers so that for every pair of distinct elements of , . Here denotes the greatest common divisor of and .
Solution
To determine if there exists a set of 2011 positive integers such that for every pair of distinct elements of , , we can proceed as follows:
1. Understanding the Condition:
The condition implies that the absolute difference between any two distinct elements and in the set must be equal to their greatest common divisor (gcd).
2. Initial Setup:
Let's start with a positive integer . We need to find a sequence of integers such that for any , divides .
3. Generating the Sequence:
Consider the sequence , where . Here, denotes the least common multiple.
4. Verifying the Condition:
For any :
Since divides , and divides , it follows that divides .
5. Ensuring the GCD Condition:
We need to ensure that . Given that divides and divides , we have:
and
Thus, the sequence satisfies the initial requirement.
6. Inductive Construction:
By induction, we can construct such sequences of arbitrary length. Starting with a base case, we can generate a sequence of length 2011 by choosing appropriate values.
Conclusion:
The construction method described above shows that it is possible to generate a set of 2011 positive integers satisfying the given condition.