Asymmetric relation

In mathematics, an asymmetric relation is a binary relation on a set X where:

• For all a and b in X, if a is related to b, then b is not related to a.

In mathematical notation, this is:

$\forall a,b\in X(aRb\Rightarrow \lnot (bRa))$

An example is the "less than" relation < between real numbers: if x < y, then necessarily y is not less than x.The "less than or equal" relation ≤, on the other hand, is not asymmetric, because reversing x ≤ x produces x ≤ x and both are true. In general, any relation in which x R x holds for some x (that is, which is not irreflexive) is also not asymmetric.

Asymmetry is not the same thing as "not symmetric": the less-than-or-equal relation is an example of a relation that is neither symmetric nor asymmetric. The empty relation is the only relation that is (vacuously) both symmetric and asymmetric.

Properties

• A relation is asymmetric if and only if it is both antisymmetric and irreflexive.
• Restrictions and inverses of asymmetric relations are also asymmetric. For example, the restriction of < from the reals to the integers is still asymmetric, and the inverse > of < is also asymmetric.
• A transitive relation is asymmetric if and only if it is irreflexive: if a R b and b R a, transitivity gives a R a, contradicting irreflexivity.
• An asymmetric relation need not be total. For example, strict subset or ⊊ is asymmetric, and neither of the sets {1,2} and {3,4} is a strict subset of the other. In general, every strict partial order is asymmetric, and conversely, every transitive asymmetric relation is a strict partial order. Not all asymmetric relations are strict partial orders. An example of an asymmetric intransitive relation is the rock-paper-scissors relation: if X beats Y, then Y does not beat X; but if X beats Y and Y beats Z, then X does not beat Z.

See also

• Tarski's axiomatization of the reals – part of this is the requirement that < over the real numbers be asymmetric.