Let be the set of positive integers. For any and in the set we have . For any , and in the set we have . Is it possible that has elements?
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Solution
To solve this problem, we need to determine if there is a set consisting of 2012 positive integers such that:
1. For any two distinct elements and in , their greatest common divisor .
2. For any three distinct elements , , and in , their greatest common divisor .
The challenge is to construct such a set of 2012 elements.
### Construction of the Set
Consider the prime numbers . We can define each element of the set as the product of two distinct primes from our chosen set of 2012 primes. Specifically, let:
for each . Let the final element be:
### Verification
1. Pairwise GCD Greater than 1:
For any two distinct elements and where , we have:
For any such and , we get:
As long as neither nor is used among the first 2011 elements (which it isn't because for ), the GCD will include at least one factor from the product, hence:
2. Triple GCD Equal to 1:
Considering any three distinct elements , , and , where two of them are from the first 2011 elements and one is :
Since the are pairwise coprime with the additional element , the GCD condition with three distinct elements holds, as their triples always share only one prime, thus:
### Conclusion
By this choice and verification, it demonstrates that such a set of 2012 elements satisfying all required conditions can be constructed. Therefore, it is indeed possible to have a set with 2012 elements following these conditions. Hence, the answer is: