Fundamental Theorem on Equivalence Relations/Examples/Arbitrary Equivalence on Set of 6 Elements 2
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Example of Use of Fundamental Theorem on Equivalence Relations
Let $S = \set {1, 2, 3, 4, 5, 6}$.
Let $\RR \subset S \times S$ be an equivalence relation on $S$ with the properties:
\(\ds 1\) | \(\RR\) | \(\ds 3\) | ||||||||||||
\(\ds 3\) | \(\RR\) | \(\ds 4\) | ||||||||||||
\(\ds 2\) | \(\RR\) | \(\ds 6\) | ||||||||||||
\(\ds \forall a \in A: \, \) | \(\ds \size {\eqclass a \RR}\) | \(=\) | \(\ds 3\) |
Then the equivalence classes of $\RR$ are:
\(\ds \eqclass 1 \RR\) | \(=\) | \(\ds \set {1, 3, 4}\) | ||||||||||||
\(\ds \eqclass 2 \RR\) | \(=\) | \(\ds \set {2, 5, 6}\) |
Proof
We have that:
- $1 \mathrel \RR 3$ and $3 \mathrel \RR 4$
As $\RR$ is an equivalence relation, it follows that $\RR$ is transitive and so:
- $1 \mathrel \RR 4$
Thus:
- $\eqclass 1 \RR \subseteq \set {1, 3, 4}$
but as:
- $\size {\eqclass a \RR} = 3$
it follows that:
- $\eqclass 1 \RR= \set {1, 3, 4}$
There are $6$ elements of $S$.
Thus the other $3$ elements must all be in the same equivalence class of $\RR$ which does not contain $1$, for example.
Thus we have:
- $\eqclass 2 \RR \subseteq \set {2, 5, 6}$
and the information we were given that $2 \mathrel \RR 6$ was superfluous.
$\blacksquare$
Sources
- 1977: Gary Chartrand: Introductory Graph Theory ... (previous) ... (next): Appendix $\text{A}.3$: Equivalence Relations: Problem Set $\text{A}.3$: $14$