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Possible Duplicates:
Symmetric, Transitive and reflexive

Why isn't reflexivity redundant in the definition of equivalence relation?

Dependence of Axioms of Equivalence Relation?

Let $X$ a set and let $\sim$ a binary relation in $X$. $\sim$ is called a equivalence relation if:

  1. $\forall x\in X$ we have $x\sim x$.
  2. $\forall x,y\in X$ if $x\sim y$ then $y\sim x$.
  3. $\forall x,y,z\in X$ if $x\sim y$ and if $y\sim z$ then $x\sim z$.

I think that 1 is unnecessary because by 2 we have that $x\sim y \Leftrightarrow y\sim x$. Then by 3. we have that $x\sim y$ and $y\sim x$ then $x\sim x$. Then 2,3 $\Rightarrow$ 1.

Am I right?

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Gaston Burrull
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  • Must I delete question or not? – Gaston Burrull Jun 01 '12 at 04:09
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    No; the question is simply closed, and a pointer to the duplicate is added. You should go read the answers there to see why you are not right. – Arturo Magidin Jun 01 '12 at 04:13
  • Yes I was full understood, then I found another duplicate "Dependence of Axioms of Equivalence Relation?" – Gaston Burrull Jun 01 '12 at 04:17
  • @ArturoMagidin In general titles are not explicit enough. Because this fact I didn't see a suggestive title. I always try to be very explicit in title as much as possible when i make a question. – Gaston Burrull Jun 01 '12 at 04:21
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    I found the first one by doing a search for reflexive transitive relation. – MJD Jun 01 '12 at 04:24
  • The explicit title is the second and third. The first one is quite not suggestive. – Gaston Burrull Jun 01 '12 at 04:29
  • The search also shows the first few lines of the body of the post. In the one I found, the first few words were "If a relation is symmetric and transitive, then it will be reflexive too. True/False?" – MJD Jun 01 '12 at 16:41

2 Answers2

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You are right unless there is some $x$ that is unrelated to the other elements. If $x\sim y$ is false for all $y$, then 2 and 3 might both hold, but 1 does not.

In particular, the empty relation, which has $x\not\sim y$ for all $x$ and $y$, is symmetric and transitive, but not reflexive.

MJD
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  • What do you mean with empty relation? – Gaston Burrull Jun 01 '12 at 04:01
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    @Gastón: this is the relation in which nothing is related to anything else. (In other words, thinking of a relation on a set $X$ as a subset of $X \times X$, this is the empty subset of $X \times X$.) – Qiaochu Yuan Jun 01 '12 at 04:01
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    An empty relation is one in which $x\sim y$ is false for all $x$ and $y$. – MJD Jun 01 '12 at 04:02
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    Not only when the relation is empty: it is enough that there's some $,x\in X,$ that isn't related to any other element. Then we *must* require reflexivity – DonAntonio Jun 01 '12 at 04:03
  • @QiaochuYuan thanks then property 2 can't be used and can't deduce 1. – Gaston Burrull Jun 01 '12 at 04:03
  • @DonAntonio Yes you're right if there is some $x$ not related to anyone then can't be deduce that $x\sim x$. – Gaston Burrull Jun 01 '12 at 04:05
  • Is surprising that the answer in this question was opposite to my other question about definition of topology http://math.stackexchange.com/questions/151924/unnecessary-property-in-definition-of-topological-space – Gaston Burrull Jun 01 '12 at 04:26
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no

what if there is no such $y$?

john
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