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Correct answer: 6
Let A relation on is a subset of , where .
We need relations such that:
- is reflexive
- is transitive
- is not symmetric
- has at most 6 elements
1. Reflexive condition
Since is reflexive, it must contain So 3 elements are already compulsory.
Also given: Thus at least 4 elements are fixed:
Since relation can have at most 6 elements, we may add at most 2 more elements from the remaining 5 pairs:
2. Not symmetric condition
Because , for symmetry we would need . So for the relation to be not symmetric, it is enough that If were included, we would still need to check other pairs; but since we want not symmetric and have a small case analysis, we will examine all valid possibilities.
3. Transitivity implications
Start with the compulsory part: This is already transitive:
- with gives , already present.
- with gives , already present.
- loops cause no issue.
So we can add up to two more pairs, but any addition must preserve transitivity.
We now test subsets of of size or .
4. Case-by-case counting
Case A: Add 0 extra pairs
Relation: It is reflexive, transitive, and not symmetric since but .
So this gives 1 relation.
Case B: Add 1 extra pair
We test each of the 5 possibilities.
(i) Add
Then . No new transitivity requirement fails. Valid.
(ii) Add
Now and imply by transitivity that is present, yes. But and imply , yes. However and don't force anything new beyond loops. This relation is actually symmetric on pair , but symmetry overall also requires reverse of every pair present. Since only non-diagonal pairs are and , it becomes symmetric. So not allowed.
(iii) Add
Then and imply must be in , but it is not. So not transitive, not allowed.
(iv) Add
Then and imply must be in , but it is not. So not transitive, not allowed.
(v) Add
No transitivity failure occurs:
- with gives .
- with gives .
- with anything starting at 2 only gives something if we had or , which we do not. Valid.
Hence Case B gives 2 valid relations:
Case C: Add 2 extra pairs
Choose 2 pairs from . There are possibilities. Check each.
1.
Since and imply , which is absent. Not transitive.
2.
Since and imply , which is present. No further issue. Not symmetric because absent. Valid.
3.
Then and imply , present; and imply , present. But and imply , absent. So not transitive.
4.
Check:
- and imply , present. No other issue. Valid.
5.
Then and imply , present. Also and imply , absent. So not transitive.
6.
Then and imply , present. and imply , absent. So not transitive.
7.
Then and imply , absent. So not transitive.
8.
Then and imply , absent. So not transitive.
9.
Then and imply , absent. Also and imply , present. But missing already fails. Not transitive.
10.
and imply , present. No further issue. Valid.
Thus Case C gives 3 valid relations.
5. Total count
Total valid relations:
So the required number of relations is
6. Comparison with stored answer
Stored correct answer = .
Our derived answer also is , so they agree.
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