# Proof: Q is symmetric and reflexive on A

Suppose that $\mathscr{P}$ is a partition of A and suppose $xQy$ if there exists $C \in \mathscr{P}$ such that $x \in C$ and $y \in C$. Prove that Q is symmetric and Q is reflexive on A.

Proof.

Let $x \in A$ and $y \in A$.

Assume that $xQy$. By definition, for all $x \in A$ and $y \in A$, if $xQy$, then $yQx$. Therefore, $\exists C \in \mathscr{P}$ such that $x \in C$ and $y \in C$. Therefore $yQx$. Hence Q is symmetric.

Let $x \in A$.

By definition, the partition of A is represented as $$A =\bigcup_{c\in \mathscr{P}}C.$$ Since $x \in A$, it follows that $x \in \bigcup_{c \in \mathscr{P}}C$. So $\exists C \in \mathscr{P}$ such that $x \in C$. Hence $xQx$. Q is reflexive.

These are my proofs. If needed, can someone correct them?

• The rest of the problem is to show that Q is transitive, ie an equivalence relation, and that the set of equivalence classes of Q is the partition P. – William Elliot Oct 20 '17 at 2:05

## 1 Answer

Let $x \in A$ and $y \in A$.

Assume that $xQy$. By definition, for all $x \in A$ and $y \in A$, if $xQy$, then $yQx$.

There's an issue here; this is not "by definition," and is in fact precisely what you're trying to conclude, so it's not OK to claim that $y Q x$ yet.

Therefore, $\exists C \in \mathscr{P}$ such that $x \in C$ and $y \in C$. Therefore $yQx$.

It might be slightly better to explicitly restate that we can conclude for this $C$ that "$y \in C$ and $x \in C$", in that order. This is obviously true, but it's the crux of why you can claim that $y Q x$. Whether or not this is imperative depends on how picky your teacher is, most likely.

Hence Q is symmetric.

Let $x \in A$.

By definition, the partition of A is represented as $$A =\bigcup_{c\in \mathscr{P}}C.$$ Since $x \in A$, it follows that $x \in \bigcup_{c \in \mathscr{P}}C$. So $\exists C \in \mathscr{P}$ such that $x \in C$.

Again, it may be worth explicitly saying that "for this $C$, we can say '$x \in C$ and $x \in C$.'" But this feels even more nitpicky than the last thing and may be unnecessary.

Hence $xQx$. Q is reflexive.

(This whole thing mostly looks good, by the way! The only serious problem is the one about prematurely claiming that $y Q x$.)