Basically, we have 4 types of relations: reflexive, symmetric, antisymmetric, transtive. And then we separate 4 above types into 2 new definition:
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One relation is reflexive, symmetric, transitve called equivalent relation.
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One relation is reflexive, antisymmetrc, transitive called order relation.
All of them above are basic knowledge of elementary set theory.
So the question I wonder is "Does there exist one relation is both reflexive, symmetric, antisymmetric and transitive? If yes, so what is it called?"
Honestly, I have been finding out in the internet about my wonder, but of course I cannot see anything. Therefore, I post my question on here to ask everyone my question.
Thanks for your helping.
Best Answer
Suppose $\sim$ is symmetric. Then for all elements $a,b$ in the ambient set $S$, we have that $a\sim b$ implies $b\sim a$. But if $\sim$ is antisymmetric, then $a\sim b$ and $b\sim a$ together imply $a=b$. Hence $\sim$ is in fact equality.