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Correct answer: 3
- Set up all required pairs
For , a relation on is a subset of .
Since is reflexive, it must contain:
It is also given that contains:
So currently,
- Use transitivity
Since and , transitivity forces:
Thus every such relation must contain at least:
- List the only remaining optional pairs
The full set has pairs. We already know of them must be present. The remaining pairs are:
Let us decide which of these can be added while keeping transitivity, and then exclude symmetric relations.
- Check all subsets of
We examine all possibilities.
Case 1: Add none
Relation contains only
This is reflexive and transitive. It is not symmetric since but .
So this case is valid.
Case 2: Add only
Now and imply by transitivity nothing new beyond reflexive pairs, but:
- and force , already present.
- and force , already present.
So transitivity is preserved. It is still not symmetric because but .
Valid.
Case 3: Add only
Then and force:
which is not present. So transitivity fails.
Invalid.
Case 4: Add only
Then and already gave . Also and force:
which is already present. And and force , already present.
So transitivity holds. But check symmetry:
- and are symmetric,
- yet but . So relation is not symmetric.
Valid.
Case 5: Add
Since and imply , already present. Since and imply:
which is missing. So transitivity fails.
Invalid.
Case 6: Add
Check transitivity:
- and imply , already present.
- and imply , which is missing. So transitivity fails.
Invalid.
Case 7: Add
Since and imply , which is missing. So transitivity fails.
Invalid.
Case 8: Add all three:
Then relation becomes all pairs of . This is reflexive, transitive, and symmetric. But we need not symmetric.
Invalid.
- Count valid relations
Valid cases are:
- none added
- only
- only
Hence the number of required relations is:
- Compare with stored answer
Derived answer = . Stored correct answer = . They agree.
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