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Why is the relation R on A irreflexive if and only if ΔA ∩ R = ∅?

I always thought the empty set is reflexive (and transitive, symmetric because it is vacuously true.)

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The empty relation is reflexive and also irreflexive. – Peter Franek Jun 22 '14 at 21:35
Thank you for your comment. I did not know that. After reading Hagen von Eitzen's comment as well and searched further, I started to understand. Thanks again. – user3125591 Jun 22 '14 at 21:56
up vote 4 down vote accepted

By definition, $R$ is irreflexive iff for all $a\in A$ we have $(a,a)\notin R$. This precisely says that the diagonal $\Delta A=\{\,(a,a)\mid a\in A\,\}$ is disjoint from $R$. Note that according to this, the empty relation is irreflexive, and it ias also (vacuously) symmetric and transitive. But the empty relation is reflexive only as relation on the empty set (i.e. $R=\emptyset$ is reflexive iff $A=\emptyset$).

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I understand it now! Very clear explanation! Thank you very much. You explained it more clear than what's in my book! – user3125591 Jun 22 '14 at 21:54

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