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Let A be A={1,2,3}, let K be the set of all symmetric and not reflexive relations of A. Is K $$ K=\{(\phi,\phi)\quad((\phi,\phi),(1,2),(2,1))\quad ((\phi,\phi),(1,3),(3,1))\quad ((\phi,\phi),(2,3),(3,2))\quad((\phi,\phi),(1,2),(2,1),(1,3),(3,1))\quad ((\phi,\phi),(1,2),(2,1),(2,3),(3,2))\quad ((\phi,\phi),(1,3),(3,1),(2,3),(3,2))\quad ((\phi,\phi),(1,2),(2,1),(1,3),(3,1),(2,3),(3,2))\} $$

???

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  • $\begingroup$ Is $K$ a group or just a set? And what is $\phi$? $\endgroup$
    – 57Jimmy
    Feb 9, 2018 at 15:39
  • $\begingroup$ @Arthur Losnikov: A relation on $A$ is a set (possibly empty) of ordered pairs of elements of $A$. But note: $\varnothing$ is not an element of $A$, so $(\varnothing,\varnothing)$ is not an ordered pair of elements of $A$, hence, it should not be in any of the relations. $\endgroup$
    – quasi
    Feb 9, 2018 at 15:42
  • $\begingroup$ ϕ is an empty set. K is a group of all possible symmetric and not reflexive relations of A. $\endgroup$ Feb 9, 2018 at 15:43
  • $\begingroup$ The word "group" should not be used (since it has another meaning). Instead, just refer to $K$ as a set, or as a collection. $\endgroup$
    – quasi
    Feb 9, 2018 at 15:45
  • $\begingroup$ K is P(AxA) here. Is it a group or a set? $\endgroup$ Feb 9, 2018 at 15:46

2 Answers 2

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If $R$ is a relation not reflexive, we cannot have $(1,1),(2,2),(3,3) \in R$ together but for example you may have $(1,1) \in R$ and $(2,2),(3,3) \notin R$. Or $(1,1),(2,2) \in R$ and $(3,3) \notin R$. So if you want to list them systematically, answer should also include the relations:

$$K=\big\{\{(1,1)\},\{(2,2)\},\{(3,3)\},\{(1,1),(2,2)\},\{(1,1),(3,3)\},\{(2,2),(3,3)\},\{(1,1),(1,2),(2,1)\},\{(1,1),(1,3),(3,1)\},\{(1,1),(2,3),(3,2)\},... \big\}$$

and none of the elements of $K$ should not include $(\emptyset,\emptyset)$ since $\emptyset$ is not an element but a representation of empty set.

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Let $K$ be the set of relations $R$ on $A$ such that $R$ is symmetric, but not reflexive.

Then $R \in K$ if and only if $R=S\cup T$, where $S$ is a proper subset of $$\{(1,1),(2,2),(3,3)\}$$ and $T$ is the union of zero or more of the three sets $$\{(1,2),(2,1)\},\{(2,3),(3,2)\},\{(3,1),(1,3)\}$$

It follows that there are $2^3-1=7$ choices for $S$, and $2^3=8$ choices for $T$, so there are $(7)(8)=56$ possibilities for $R$.

Thus, $K$ has $56$ elements.

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  • $\begingroup$ What is the least element of this K set? $\endgroup$ Feb 9, 2018 at 18:12
  • $\begingroup$ The empty set is an element of $K$, so that qualifies as a kind of "least" element. But $K$ is not a set of numbers. It's a set of sets (since relations on $A$ are subsets of $A\times A$). $\endgroup$
    – quasi
    Feb 9, 2018 at 18:15
  • $\begingroup$ To form an element of $R$ of $K$, choose any proper subset $S$ of $$\{(1,1),(2,2),(3,3)\}$$ and let $T$ be the union of zero or more of the three sets $$\{(1,2),(2,1)\},\{(2,3),(3,2)\},\{(3,1),(1,3)\}$$and then let $R=S\cup T$. $\endgroup$
    – quasi
    Feb 9, 2018 at 18:22
  • $\begingroup$ Then why the empty set is an element of K if its not there? $\endgroup$ Feb 9, 2018 at 18:22
  • $\begingroup$ The empty set is a subset of $A\times A$. As a relation, the empty set is symmetric, but not reflexive, so it qualifies as an element of $K$. $\endgroup$
    – quasi
    Feb 9, 2018 at 18:23

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