Equivalence Relation/Examples/Non-Equivalence/Is the Sister Of
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Example of Relation which is not Equivalence
Let $P$ denote the set of people.
Let $\sim$ be the relation on $P$ defined as:
- $\forall \tuple {x, y} \in P \times P: x \sim y \iff \text { $x$ is the sister of $y$}$
Then $\sim$ is not an equivalence relation.
Proof
For a start, no person can be his or her own sister, so:
- $\forall x: x \nsim x$
So $\sim$ is not reflexive.
Then:
- If $x \sim y$ then it is not necessarily the case that $y$ is the sister of $x$.
This is because $y$ may be male, and so would be the brother of $x$.
So $\sim$ is not symmetric.
Let us assume that $\sim$ specifically means has the same father and mother as, and does not encompass half-siblings.
Thus:
- if $x \sim y$ and $y \sim z$ it follows that $x$ is the sister of $z$.
So $\sim$ is transitive.
$\blacksquare$
Sources
- 1977: Gary Chartrand: Introductory Graph Theory ... (previous) ... (next): Appendix $\text{A}.2$: Cartesian Products and Relations: Problem Set $\text{A}.2$: $7 \ \text{(c)}$