Element uniqueness/antisymmetry in partial ordering

elementary-set-theoryequivalence-relationsorder-theoryrelations

  1. Consider set $A = \{a, b, c \}$ and suppose that our relations are aRa, bRb, and cRc.

I believe this is a partial ordering because all of the relations follow reflexivity, transitivity, and antisymmetry. Would that be correct?

  1. Consider set $A = \{a, b, c \}$ and suppose that our relations are aRa, aRb, bRa, bRb, cRc.

I am confused about whether this is a partial ordering. In particular, bRa, aRb means that a = b by antisymmetry. Since the set definition does not reflect that $a = b$, does this relation not satisfy the criteria for a partial ordering?

Best Answer

The second relation is not a partial order, because it fails antisymmetry, but it is an equivalence relation (because it is reflexive, symmetric, and transitive, but not anti-symmetric, for the reason you state: $aRb$ and $bRa$, but $a \neq b$).

The first relation is both an equivalence relation and, as you note, a partial order. (It is vacuously symmetric and antisymmetric and transitive).


Note: A set, e.g., $A= \{a, b, c\}$ is such that there is no duplicity of elements. So, e.g., you can rest assured that $a \neq b$, else the set would be $A= \{a, c\},$, or $A= \{b, c\}$. By the definition of a set, every listed in the set, is distinct wrt other elements in the set.

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