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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 and only if:
If RT represents the converse of R, then R is symmetric if and only if R = RT.
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 conceptualize a symmetric relation in graph theory is that a symmetric relation is an edge, with the edge's two vertices being the two entities so related. Thus, symmetric relations and undirected graphs are combinatorially equivalent objects.