# Relation $\rho$ such that $a = bn$ not anti-symmetric?

I have a homework question that asks me to determine determine whether given relations are reflexive, symmetric, anti-symmetric, asymmetric or transitive.

One relation is:

$\rho \subseteq \mathbb \times \mathbb Z$, where $a~\rho~b$ if and only if there is $n \in \mathbb Z$ such that $a = bn$

The answer says it's reflexive and transitive. How come is it not anti-symmetric?

We only find $x~\rho~y$ and $y~\rho~x$ if $x = y$.

like in (1,1) or (2,2).

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• Domain is integers, not natural numbers: $(1,-1)$ as well as $(-1,1)$. – Hendrik Jan Sep 23 '16 at 22:52
• thank you this is it! You can put it as an answer (as a counter example that proves anti-symmetry false) @HendrikJan – Mina Michael Sep 23 '16 at 22:59

## 1 Answer

Take $a=0,b=5$. Then $a~\rho~b$ (take $n=0$) and $b~\rho~a$ (take $n=0$). However, $a\neq b$.

Antisymmetry requires that if $a~\rho~b$ and $b~\rho~a$ for some $a,b$, it must hold that $a=b$. Since the antecedent is true in the above example, but the consequent is not, $\rho$ cannot be antisymmetric.

• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Qwerty Sep 24 '16 at 9:30
• @Qwerty it does provide an answer; it shows why $\rho$ is not antisymmetric by providing a counter argument. I will edit to make that clearer. – user63495 Sep 24 '16 at 9:37