A binary relation $\mathcal{R}$ on a set $A$ is said to be irreflexive (or antireflexive) if $\forall a\in A$ , $\neg a\mathcal{R} a$ . In other words, ``no element is $\mathcal{R}$ -related to itself."

For example, the relation $<$ (``less than") is an irreflexive relation on the set of natural numbers.

Note that ``irreflexive" is not simply the negation of ``reflexive ." Although it is impossible for a relation (on a nonempty set) to be both reflexive and irreflexive, there exist relations that are neither. For example, the relation $\{(a,a)\}$ on the two element set $\{a,b\}$ is neither reflexive nor irreflexive.

Here is an example of a non-reflexive, non-irreflexive relation ``in nature." A subgroup in a group is said to be self-normalizing if it is equal to its own normalizer. For a group $G$ , define a relation $\mathcal{R}$ on the set of all subgroups of $G$ by declaring $H\mathcal{R}K$ if and only if $H$ is the normalizer of $K$ . Notice that every nontrivial group has a subgroup that is not self-normalizing; namely, the trivial subgroup $\{e\}$ consisting of only the identity. Thus, in any nontrivial group $G$ , there is a subgroup $H$ of $G$ such that $\neg H\mathcal{R} H$ . So the relation $\mathcal{R}$ is non-reflexive. Moreover, since the normalizer of a group $G$ in $G$ is $G$ itself, we have $G\mathcal{R} G$ . So $\mathcal{R}$ is non-irreflexive.