Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2005/calculus-linear-algebra-discrete-mathematics-logic-and-proof-and-models-of-computation/7/b/solution

The requested examples are binary relations on sets, rather than partially ordered sets: a partially ordered set necessarily has all three properties. This corrects the wording of the source.[ref]
For an antisymmetric relation and transitive relation that is not a reflexive relation, take and . No distinct pair relates both ways, so the antisymmetric relation condition holds. There are no composable pairs, so the transitive relation condition holds vacuously. Neither nor belongs to , so the reflexive relation condition fails.
For a reflexive relation and transitive relation that is not an antisymmetric relation, take and . All diagonal pairs and all composite pairs belong to , but and both hold with .
For a reflexive relation and antisymmetric relation that is not a transitive relation, take
All diagonal pairs are present, and no off-diagonal pair has its reverse present. However and hold while fails. Thus each pair of axioms can hold without the third.

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