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A symmetric relation is a type of binary relation. An example is the relation "is equal to", because if a = b is true then b = a is also true. Formally, a binary relation R over a set X is symmetric if:
where the notation means that .
- "is equal to" (equality) (whereas "is less than" is not symmetric)
- "is comparable to", for elements of a partially ordered set
- "... and ... are odd":
- "is married to" (in most legal systems)
- "is a fully biological sibling of"
- "is a homophone of"
- "is co-worker of"
- "is teammate of"
Relationship to asymmetric and antisymmetric relations
By definition, a nonempty relation cannot be both symmetric and asymmetric (where if a is related to b, then b cannot be related to a (in the same way)). However, a relation can be neither symmetric nor asymmetric, which is the case for "is less than or equal to" and "preys on").
Symmetric and antisymmetric (where the only way a can be related to b and b be related to a is if a = b) are actually independent of each other, as these examples show.
|Antisymmetric||equality||"is less than or equal to"|
|Not antisymmetric||congruence in modular arithmetic||"is divisible by", over the set of integers|
|Antisymmetric||"is the same person as, and is married"||"is the plural of"|
|Not antisymmetric||"is a full biological sibling of"||"preys on"|
- One way to count the symmetric relations on n elements, that in their binary matrix representation the upper right triangle determines the relation fully, and it can be arbitrary given, thus there are as many symmetric relations as nxn binary upper triangle matrices, 
|Elements||Any||Transitive||Reflexive||Symmetric||Preorder||Partial order||Total preorder||Total order||Equivalence relation|
|n||2n2||2n2−n||2n(n+1)/2||S(n, k)||n!||S(n, k)|
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