MHB Help with an Equivalence Relation?

AI Thread Summary
The discussion centers on determining whether the relation defined by aRb if and only if |a| = |b| on the real numbers is an equivalence relation. It is established that the relation is reflexive since |a| equals |a| for all a in ℝ. Additionally, a theorem is referenced that confirms a relation defined through a function can be an equivalence relation. The conversation also touches on the limitations of defining relations outside of functions, highlighting that not all arbitrary relations satisfy equivalence properties. Overall, the participants clarify the nature of the equivalence relation in question.
katye333
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Hello all, I have an equivalence relation that I need some help with. Normally I find these to be fairly simple, however I'm not sure if I'm over-thinking this one or if it's just tricky.

For the relation: aRb $\Longleftrightarrow$ |a| = |b| on $\mathbb{R}$ determine whether it is an equivalence relation.

Reflexive: Would it really be reflexive? If a = -2, then wouldn't |a| = +2?

Or would it be reflexive, since all a's are contained in a?
 
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katye333 said:
Reflexive: Would it really be reflexive? If a = -2, then wouldn't |a| = +2?
Or would it be reflexive, since all a's are contained in a?

A relation $R$ is reflexive on a set $S$ if and only if $aRa$ for all $a\in S$. In our case $|a|=|a|$ for all $a\in \mathbb{R}$ i.e. $aRa$ for all $a\in \mathbb{R}$, which implies that $R$ is reflexive on $\mathbb{R}$.
 
You can prove a general theorem. Let $A$, $B$ be sets and $f:A\to B$ a total function. Let a relation $R$ on $A$ be defined by $a_1Ra_2\iff f(a_1)=f(a_2)$. Then $R$ is an equivalence relation. In this case $f:\mathbb{R}\to\mathbb{R}$ is the absolute value function. You can also apply this theorem to $f(x)=\mathop{\text{sgn}}(x)=\begin{cases}1&x>0\\ 0&x=0\\ -1&x<0\end{cases}$, $f(x)=\lfloor x\rfloor$, etc.

Note also that if we take not a function $f$, but an arbitrary relation $F\subseteq A\times B$ and define \[a_1Ra_2\iff \text{there exists a }b\in B\text{ such that }a_1Fb\text{ and }a_2Fb\] then the statement does not hold in general. (Which property of an equivalence relation gets violated?) For example, it does not hold if $A=B$ is a set of people and $aFb\iff b$ is a friend $a$.
 
Thank you both for the responses.
I don't know why that one confused me, while none of the others did. :p
 

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