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- (Reflexivity) a ~ a
- (Symmetry) if a ~ b then b ~ a
- (Transitivity) if a ~ b and b ~ c then a ~ c
A set together with an equivalence relation is called a setoid.
Equivalence relations are often used to group together objects that are similar in some sense.
Examples of equivalence relations
- The equality ("=") relation between real numbers or sets.
- The relation "is congruent to (modulo 5)" between integers.
- The relation "is similar to" on the set of all triangles.
- The relation "has the same birthday as" on the set of all human beings.
- The relation of logical equivalence on statements in first-order logic.
- The relation "is isomorphic to" on models of a set of sentences.
- The relation "is in thermal equilibrium with".
- Green's relations are five equivalence relations on the elements of a semigroup.
Examples of relations that are not equivalences
- The relation "≥" between real numbers is not an equivalence relation, because although it is reflexive and transitive, it is not symmetric. E.g. 7 ≥ 5 does not imply that 5 ≥ 7.
- The relation "has a common factor with" between natural numbers is not an equivalence relation, because although it is reflexive and symmetric, it is not transitive (2 and 6 have a common factor greater than 1, and 6 and 3 have a common factor greater than 1, but 2 and 3 do not have a common factor greater than 1).
- The empty relation R on a non-empty set X (i.e. a R b is never true) is not an equivalence relation, because although it is vacuously symmetric and transitive, it is not reflexive (except when X is also empty).
- The relation "is approximately equal" between real numbers or other things, even if more precisely defined, is not an equivalence relation, because although it is reflexive and symmetric, it is not transitive (it may seem so at first sight, but many small changes can add up to a big change).
- The relation "is the mother of" on the set of all human beings is not an equivalence relation, because it not reflexive (A is not the mother of A), symmetric (If A is the mother of B, then B is not the mother of A), and is not transitive (if A is the mother of B, and B is the mother of C, it does not necessarily mean A is the mother of C)
Partitioning into equivalence classes
Every equivalence relation on X defines a partition of X into subsets called equivalence classes: all elements equivalent to each other are put into one class. Conversely, if the set X can be partitioned into subsets, then we can define an equivalence relation ~ on X by the rule "a ~ b if and only if a and b lie in the same subset".
For example, if G is a group and H is a subgroup of G, then we can define an equivalence relation ~ on G by writing a ~ b if and only if ab-1 lies in H. The equivalence classes of this relation are the right cosets of H in G.
If an equivalence relation ~ on X is given, then the set of all its equivalence classes is the quotient set of X by ~ and is denoted by X/~.
Generating equivalence relations
If two equivalence relations over the set X are given, then their intersection (viewed as subsets of X×X) is also an equivalence relation. This allows for a convenient way of defining equivalence relations: given any binary relation R on X, the equivalence relation generated by R is the smallest equivalence relation containing R.
Concretely, the equivalence relation ~ generated by R can be described as follows: a ~ b if and only if there exist elements x1, x2,...,xn in X such that x1 = a, xn = b and such that (xi,xi+1) or (xi+1,xi) is in R for every i = 1,...,n-1.
Note that the resulting equivalence relation can often be trivial! For instance, the equivalence relation ~ generated by the binary relation ≤ has exactly one equivalence class: x~y for all x and y. More generally, the equivalence relation will always be trivial when generated on a relation R having the "antisymmetric" property that, given any x and y, either x R y or y R x must be true.
One often generates equivalence relations to quickly construct new spaces by "gluing things together". Consider for instance the square X = [0,1]x[0,1] and the equivalence relation on X generated by the requirements (a,0) ~ (a,1) for all a in [0,1] and (0,b) ~ (1,b) for all b in [0,1]. Then the quotient space X/~ can be naturally identified with a torus: take a square piece of paper, bend it to glue together the upper and lower edge, then bend the resulting cylinder to glue together the two mouths.
Common notions in Euclid's Elements
Common Notion 1. Things which equal the same thing also equal one another.
Nowadays, a binary relation is called Euclidean if it satisfies this property.
Unfortunately, he didn't mention symmetry or reflexivity. But this suggests an alternative formulation: An equivalence relation is a relation which is Euclidean, symmetric and reflexive.
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