# What is the standard notation for a set of equivalence classes?

What is the standard notation for a set of equivalence relations? Specifically, I have a pair of objects, call them $x$ and $y$ and I denote the ordered pair as $\left(x,y\right)$. I have a set of equivalence relations such as:

$\left(x,y\right) \sim \left(x^{-1},y\right) \sim \left(x,y^{-1}\right) \sim \left(y^{-1},x^{-1}\right)$

I would like to write this compactly, but I'm unsure of what the standard notation would be. Is the following appropriate?

$\left\{\left(x,y\right),\left(x^{-1},y\right),\left(x,y^{-1}\right),\left(y^{-1},x^{-1}\right)\right\}$

-
I think the usual way is to write just what you have written (perhaps followed by "$\forall x, y \in G$"), and say the quotient is $G/\sim$. – Kundor Apr 23 '13 at 17:20

You can represent an equivalence class by using a representative from the class, and denoting the entire class by, say, $[(a, b)]$: this represents the set of all ordered pairs $(x, y)$ such that $(x, y) \sim (a, b)$.

So, for a given equivalence relation denoted by $\sim$: one of its equivalence classes can be denoted: $$[(a, b)] = \{(x, y)\mid (x, y) \sim (a, b)\}$$

If there are many equivalence classes determined by an equivalence relation, and you want to denote the set of equivalence classes, you can list the equivalence classes as elements of a set:

• $\{[(a, b)], [(c,d)], [(e, f)], \cdots [(y, z)]\}$ if there is a finite set of them.
• For example, the set of equivalence classes determined by the equivalence relation of congruence modulo $4$ on the set of integers, you could write $\{[0], [1], [2], [3]\}$,
• or, in the case of an infinite number of equivalence classes, like those corresponding to equality/identity on the natural numbers, one can write $\{[1], [2], [3], \cdots \}$.

If you are asking how to denote a set of different equivalence relations, where the elements of the set are relations, I'm not aware of the standard notation. (That's not to say it doesn't exist.)

-
What I want to do is have a way to denote the entire set of equivalence relations, like $P=\left\{\left(x,y\right),\left(x^{-1},y\right),\left(x,y^{-1}\right),\left(y^{‌​-1},x^{-1}\right)\right\}$, so that I can later define the quotient set as $G/P$, so I need a way to denote the entire set of equivlanece relations not just one of them. – okj Apr 22 '13 at 19:27
Try using square brackets to denote that the elements of $P$ are equivalence classes:$$P=\left\{\left[\left(x,y\right)\right],\left[\left(x^{-1},y\right)\right],\left[‌​\left(x,y^{-1}\right)\right],\left[\left(y^{‌​-1},x^{-1}\right)\right]\right\}$$ – amWhy Apr 22 '13 at 21:38
@amWhy: Very nice answer +1 – Amzoti Apr 23 '13 at 0:17

If $\sim$ is an equivalence class over $A$, then in many places we write $A/{\sim}$ as the set of equivalence classes.

This notation is similar, and on purpose, to the notation from algebra when writing $V/W$ for the quotient subspace, or $G/H$ for the quotient group, or $R/I$ when taking a quotient of a ring by an ideal.

The reason is that all those quotients actually induce an equivalence relation, and we have a natural structure on the set of equivalence classes.

-
I honestly don't know what could be wrong in this answer, and I would be very glad if someone would tell me. Perhaps something is wrong with the user posting it instead... – Asaf Karagila Dec 16 '13 at 21:30
– Daniel Fischer Dec 17 '13 at 10:43