If is a finite set, let denote the number of elements in . Call an ordered pair of subsets of if for each , and for each . How many admissible ordered pairs of subsets are there? Prove your answer.
Problem 1336
Official solution
To solve the problem, we need to count the number of admissible ordered pairs of subsets of such that for each and for each .
1. Understanding the conditions:
- For each element , .
- For each element , .
2. Analyzing the constraints:
- If is non-empty, let . Then, for each , . This means can only contain elements greater than .
- Similarly, if is non-empty, let . Then, for each , . This means can only contain elements greater than .
3. **Considering the possible values for and **:
- If and are both non-empty, the smallest element in must be greater than the size of , and the smallest element in must be greater than the size of . This creates a contradiction unless both sets are empty.
4. Counting the admissible pairs:
- The only way to satisfy both conditions without contradiction is if both and are empty. Therefore, the only admissible pair is .
5. Conclusion:
- There is exactly one admissible ordered pair of subsets of .
The final answer is .