# Partial equivalence relation

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In mathematics, a partial equivalence relation (often abbreviated as PER, in older literature also called restricted equivalence relation) $R$ on a set $X$ is a relation that is symmetric and transitive. In other words, it holds for all $a, b, c \in X$ that:

1. if $a R b$, then $b R a$ (symmetry)
2. if $a R b$ and $b R c$, then $a R c$ (transitivity)

If $R$ is also reflexive, then $R$ is an equivalence relation.

## Properties and applications

In a set-theoretic context, there is a simple structure to the general PER $R$ on $X$: it is an equivalence relation on the subset $Y = \{ x \in X | x\,R\,x\} \subseteq X$. ($Y$ is the subset of $X$ such that in the complement of $Y$ ($X\setminus Y$) no element is related by $R$ to any other.) By construction, $R$ is reflexive on $Y$ and therefore an equivalence relation on $Y$. Notice that $R$ is actually only true on elements of $Y$: if $x R y$, then $y R x$ by symmetry, so $x R x$ and $y R y$ by transitivity. Conversely, given a subset Y of X, any equivalence relation on Y is automatically a PER on X.

PERs are therefore[why?] used mainly in computer science, type theory and constructive mathematics, particularly to define setoids, sometimes called partial setoids. The action of forming one from a type and a PER is analogous to the operations of subset and quotient in classical set-theoretic mathematics.

Every partial equivalence relation is a difunctional relation, but the converse does not hold.

The algebraic notion of congruence can also be generalized to partial equivalences, yielding the notion of subcongruence, i.e. a homomorphic relation that is symmetric and transitive, but not necessarily reflexive.[1]

## Examples

A simple example of a PER that is not an equivalence relation is the empty relation $R=\emptyset$ (unless $X=\emptyset$, in which case the empty relation is an equivalence relation (and is the only relation on $X$)).

### Euclidean parallelism

In the Euclidean plane, two lines m and n are parallel lines when mn = ∅. The symmetry of this relation is obvious and the transitivity can be proven in the Euclidean plane, thus Euclidean parallelism is a partial equivalence relation. Nevertheless, mathematicians developing affine geometry prefer the facility of an equivalence relation and therefore sometimes revise the definition of parallelism to allow a line to be parallel to itself, making the new relation of "affine parallelism" that is a reflexive relation.

### Kernels of partial functions

For another example of a PER, consider a set $A$ and a partial function $f$ that is defined on some elements of $A$ but not all. Then the relation $\approx$ defined by

$x \approx y$ if and only if $f$ is defined at $x$, $f$ is defined at $y$, and $f(x) = f(y)$

is a partial equivalence relation but not an equivalence relation. It possesses the symmetry and transitivity properties, but it is not reflexive since if $f(x)$ is not defined then $x \not\approx x$ — in fact, for such an $x$ there is no $y \in A$ such that $x \approx y$. (It follows immediately that the subset of $A$ for which $\approx$ is an equivalence relation is precisely the subset on which $f$ is defined.)

### Functions respecting equivalence relations

Let X and Y be sets equipped with equivalence relations (or PERs) $\approx_X, \approx_Y$. For $f,g : X \to Y$, define $f \approx g$ to mean:

$\forall x_0 \; x_1, \quad x_0 \approx_X x_1 \Rightarrow f(x_0) \approx_Y g(x_1)$

then $f \approx f$ means that f induces a well-defined function of the quotients $X / \approx_X \; \to \; Y / \approx_Y$. Thus, the PER $\approx$ captures both the idea of definedness on the quotients and of two functions inducing the same function on the quotient.

## References

1. ^ J. Lambek (1996). "The Butterfly and the Serpent". In Aldo Ursini, Paulo Agliano. Logic and Algebra. CRC Press. pp. 161–180. ISBN 978-0-8247-9606-8.