# Find the equivalence class of this relation

The set R:

$$R =$$ {(0,1), (0,0), (1,0), (1,1), (2,3), (2,2), (3,2), (3,3) (4,5), (5,4), (4,4), (6,6), (5,5) (6,7), (7,6), (7,7)}

I was just wondering if I've found the right equivalence classes for this relation?

My working:

The relation is an equivalence relation. Therefore, $$[X]_R =$${ {0,1}, {2,3}, {4,5}, {6,7} }

• $R$ is not an equivalence relation because it is not transitive. $(1,0)\in R$ and $(0,1)\in R$, but $(1,1)\notin R$. – user729424 Jan 20 '20 at 23:35
• @AndrewOstergaard sorry turns out there was a typo, I just corrected it. – jame_smith Jan 20 '20 at 23:40
• "The relation is an equivalence relation. Therefore, [X]R={ {0,1}, {2,3}, {4,5}, {6,7} }" Huh???? Why does R being an equivalence relation mean those are the classes? – fleablood Jan 22 '20 at 2:37
• Also what set is $R$ a relation on? If it's a relation on any set other than $\{0,1,2,3,4,5,6,7\}$ its not an equivalence relation. – fleablood Jan 22 '20 at 2:39

Yes. $$X/R = \{\{0,1\},\{2,3\},\{4,5\},\{6,7\}\}$$
The first task is to confirm that $$R$$ is an equivalence relation over $$\{0,1,2,3,4,5,6,7\}$$ (that is $$X$$).   After your correction, we can quickly verify that $$R$$ is indeed reflexive, symmetric, and transitive over $$X$$.   After a quick resort, we can just eyeball it.
$${R}~{=}~{\{~{{(0,0), (0,1), (1,0), (1,1),}\\{ (2,2), (2,3), (3,2), (3,3),}\\{ (4,4), (4,5), (5,4), (5,5),}\\{ (6,6), (6,7), (7,6), (7,7)}}~\}}$$
The next task is to identify what subsets sets of the base set are related. That is, determine how $$R$$ partitions the set.   We only need to step through all of the eight elements of $$X$$ and sort them into sets of related elements.   We immediately see that $$0$$ is related only to itself and $$1$$, and so forth.
Thus this partition is indeed: $$\{\{0,1\},\{2,3\},\{4,5\},\{6,7\}\}$$.
This partition is the set of the equivalence classes of $$R$$ over $$X$$.