Does the set contain a subset consisting of 2000 numbers that implies ?!! :?:
Solution
Let's consider the set . We are tasked with determining whether there exists a subset such that and for every element , the element .
To solve this problem, we first think about the constraints imposed by the requirement . This means that for each included in the subset , its double cannot also be in .
1. Counting the Valid Elements:
- A number in set can have its double in the same set provided .
- The largest integer for which is .
2. Construction of Subset:
- We can construct such that contains as many numbers as possible, none of which have their doubles also included.
- We start by considering the largest possible numbers in which do not have their doubles in . These are .
3. Counting the Available Space:
- The subset consists of elements.
- This subset satisfies the condition because the smallest element when doubled is greater than 3000, hence not in .
4. **Attempt to Extend :**
- We need at least 2000 elements in . The previous subset only has 1500 elements.
- To achieve 2000 elements, any additional elements must come from the set without violating the condition .
5. Limitations:
- Selecting any value from means its double also has the potential to be in , specifically in .
- This constraint makes it impossible to construct a valid set of size 2000, as any expansion beyond the initial 1500 would immediately conflict with the condition.
Hence, the answer to the question is:
This solution demonstrates that it is not possible to construct a subset that contains 2000 elements while satisfying the condition that no element has its double also in the subset.