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Correct answer: 960
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Let the set be A relation on is a subset of .
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For a relation to be symmetric, whenever is in the relation, must also be in the relation.
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Count all symmetric relations on .
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Diagonal pairs are: Each of these can be chosen independently, since each is automatically symmetric. So number of choices for diagonal pairs is
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Off-diagonal pairs come in unordered symmetric pairs: There are such pairs. For each such pair, either both are included or both are excluded. So number of choices for off-diagonal part is
Hence total number of symmetric relations is
- Now subtract the symmetric relations that are reflexive.
A relation is reflexive if all diagonal pairs are present.
So for reflexive symmetric relations:
- Diagonal entries are fixed: only choice.
- Off-diagonal symmetric pairs still have choices.
Thus number of reflexive symmetric relations is
- Therefore, the number of symmetric relations that are not reflexive is
So the required answer is
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