Why is a symmetric relation defined: $forall xforall y( xRyimplies yRx)$ and not $forall xforall y (xRyiff yRx)$? Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)How to prove relation is asymmetric if it is both anti-symmetric and irreflexiveIs an Anti-Symmetric Relation also Reflexive?Graph, Relation $xRy Leftrightarrow$ There is a path between $x$ and $y$ - symmetryDefine symmetric relation R on set SProve that if $R$ is a symmetric, transitive relation on $A$ and the domain of $R$ is $A$, then $R$ is reflexive on $A$.If R is symmetric, must $S=forall xin X exists yin Y (xRy) $ be symmetric?Given set A, is the relation A x A always anti symmetric?Definition of symmetric relationName for relation property: If $xRy$ and $xRz$ and $x not =y$, then $yRz$.Can a relation be transitive when it is symmetric but not reflexive?

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Why is a symmetric relation defined: $forall xforall y( xRyimplies yRx)$ and not $forall xforall y (xRyiff yRx)$?



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)How to prove relation is asymmetric if it is both anti-symmetric and irreflexiveIs an Anti-Symmetric Relation also Reflexive?Graph, Relation $xRy Leftrightarrow$ There is a path between $x$ and $y$ - symmetryDefine symmetric relation R on set SProve that if $R$ is a symmetric, transitive relation on $A$ and the domain of $R$ is $A$, then $R$ is reflexive on $A$.If R is symmetric, must $S=forall xin X exists yin Y (xRy) $ be symmetric?Given set A, is the relation A x A always anti symmetric?Definition of symmetric relationName for relation property: If $xRy$ and $xRz$ and $x not =y$, then $yRz$.Can a relation be transitive when it is symmetric but not reflexive?










7












$begingroup$


Why is a symmetric relation defined by $forallxforally(xRy implies yRx)$ and not $forallxforally(xRy iff yRx)$?
(I have only found a couple of sources that defines it with a biconditional)



For example, according to Wolfram:




A relation $R$ on a set $S$ is symmetric provided that for every $x$ and $y$ in $S$ we have $xRy iff yRx$.




But the majority of books defines it the other way.
And I think I agree with the second definition.



Because if we use the first definition with "$implies$", we know the truth table of the implication in particular $P implies Q$ is true when $P$ is false and $Q$ is true. That means in the context of symmetric relation that $(x,y) notin R implies (y,x) in R$ is true.



And the example $A = 1,2,3,4$ with relation $R = (2,1),(3,1),(4,1)$ satisfies the definition because $(x,y) notin R implies (y,x) in R$ is true.



And for me it's weird that this case is considered symmetric.
Or maybe I have a profound confusion with the concept.
I would like that you guys help me clarify. *Sorry for my grammar I'm not a native english speaker.










share|cite|improve this question











$endgroup$











  • $begingroup$
    It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
    $endgroup$
    – fleablood
    Mar 23 at 15:34






  • 1




    $begingroup$
    "That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
    $endgroup$
    – fleablood
    Mar 23 at 15:44






  • 2




    $begingroup$
    The definitions are equivalent.
    $endgroup$
    – PyRulez
    Mar 23 at 18:56















7












$begingroup$


Why is a symmetric relation defined by $forallxforally(xRy implies yRx)$ and not $forallxforally(xRy iff yRx)$?
(I have only found a couple of sources that defines it with a biconditional)



For example, according to Wolfram:




A relation $R$ on a set $S$ is symmetric provided that for every $x$ and $y$ in $S$ we have $xRy iff yRx$.




But the majority of books defines it the other way.
And I think I agree with the second definition.



Because if we use the first definition with "$implies$", we know the truth table of the implication in particular $P implies Q$ is true when $P$ is false and $Q$ is true. That means in the context of symmetric relation that $(x,y) notin R implies (y,x) in R$ is true.



And the example $A = 1,2,3,4$ with relation $R = (2,1),(3,1),(4,1)$ satisfies the definition because $(x,y) notin R implies (y,x) in R$ is true.



And for me it's weird that this case is considered symmetric.
Or maybe I have a profound confusion with the concept.
I would like that you guys help me clarify. *Sorry for my grammar I'm not a native english speaker.










share|cite|improve this question











$endgroup$











  • $begingroup$
    It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
    $endgroup$
    – fleablood
    Mar 23 at 15:34






  • 1




    $begingroup$
    "That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
    $endgroup$
    – fleablood
    Mar 23 at 15:44






  • 2




    $begingroup$
    The definitions are equivalent.
    $endgroup$
    – PyRulez
    Mar 23 at 18:56













7












7








7


2



$begingroup$


Why is a symmetric relation defined by $forallxforally(xRy implies yRx)$ and not $forallxforally(xRy iff yRx)$?
(I have only found a couple of sources that defines it with a biconditional)



For example, according to Wolfram:




A relation $R$ on a set $S$ is symmetric provided that for every $x$ and $y$ in $S$ we have $xRy iff yRx$.




But the majority of books defines it the other way.
And I think I agree with the second definition.



Because if we use the first definition with "$implies$", we know the truth table of the implication in particular $P implies Q$ is true when $P$ is false and $Q$ is true. That means in the context of symmetric relation that $(x,y) notin R implies (y,x) in R$ is true.



And the example $A = 1,2,3,4$ with relation $R = (2,1),(3,1),(4,1)$ satisfies the definition because $(x,y) notin R implies (y,x) in R$ is true.



And for me it's weird that this case is considered symmetric.
Or maybe I have a profound confusion with the concept.
I would like that you guys help me clarify. *Sorry for my grammar I'm not a native english speaker.










share|cite|improve this question











$endgroup$




Why is a symmetric relation defined by $forallxforally(xRy implies yRx)$ and not $forallxforally(xRy iff yRx)$?
(I have only found a couple of sources that defines it with a biconditional)



For example, according to Wolfram:




A relation $R$ on a set $S$ is symmetric provided that for every $x$ and $y$ in $S$ we have $xRy iff yRx$.




But the majority of books defines it the other way.
And I think I agree with the second definition.



Because if we use the first definition with "$implies$", we know the truth table of the implication in particular $P implies Q$ is true when $P$ is false and $Q$ is true. That means in the context of symmetric relation that $(x,y) notin R implies (y,x) in R$ is true.



And the example $A = 1,2,3,4$ with relation $R = (2,1),(3,1),(4,1)$ satisfies the definition because $(x,y) notin R implies (y,x) in R$ is true.



And for me it's weird that this case is considered symmetric.
Or maybe I have a profound confusion with the concept.
I would like that you guys help me clarify. *Sorry for my grammar I'm not a native english speaker.







discrete-mathematics logic definition relations






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Mar 23 at 21:44









Asaf Karagila

309k33441775




309k33441775










asked Mar 23 at 15:17









Rodrigo SangoRodrigo Sango

1376




1376











  • $begingroup$
    It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
    $endgroup$
    – fleablood
    Mar 23 at 15:34






  • 1




    $begingroup$
    "That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
    $endgroup$
    – fleablood
    Mar 23 at 15:44






  • 2




    $begingroup$
    The definitions are equivalent.
    $endgroup$
    – PyRulez
    Mar 23 at 18:56
















  • $begingroup$
    It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
    $endgroup$
    – fleablood
    Mar 23 at 15:34






  • 1




    $begingroup$
    "That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
    $endgroup$
    – fleablood
    Mar 23 at 15:44






  • 2




    $begingroup$
    The definitions are equivalent.
    $endgroup$
    – PyRulez
    Mar 23 at 18:56















$begingroup$
It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
$endgroup$
– fleablood
Mar 23 at 15:34




$begingroup$
It's for ALL x, y. In your example $(2,1)in R not implies (1,2)in R$. But given $A=1,2,3$ and $R=(1,2),(2,1)$ we have $(1,3)in R implies (3,1)in R$ etc.
$endgroup$
– fleablood
Mar 23 at 15:34




1




1




$begingroup$
"That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
$endgroup$
– fleablood
Mar 23 at 15:44




$begingroup$
"That means in the context of symmetric relation that (x,y)∉ R ⟹ (y,x)∈ R is true" Only if $x,y$ is actually in $R$. $(2,1)not in R implies (1,2)in R$ is a true statement. But $(3,2)not in R implies (2,3)in R$ is a false statement.
$endgroup$
– fleablood
Mar 23 at 15:44




2




2




$begingroup$
The definitions are equivalent.
$endgroup$
– PyRulez
Mar 23 at 18:56




$begingroup$
The definitions are equivalent.
$endgroup$
– PyRulez
Mar 23 at 18:56










4 Answers
4






active

oldest

votes


















7












$begingroup$

For all $x$ and all $y$ make the if and only if unnecessary (albeit perfectly acceptable).



1) $(x,y) in R implies (y,x) in R$ for ALL $x,y in A$



And the statement 2) $(x,y) in R iff (y,x) in R$ are equivalent statements.



If 1) is true and $(x,y) not in R$ then although $(x,y)in Rimplies (y,x)in R$ or $F implies (y,x)in R$ is true, it does not tell us any thing about whether or not $(y,x) in R$. However $(y,x) in R implies (x,y) in Y$ tells us that $(y,x) not in R$. Because $(y,x) in R implies (x,y) in R$ means $(y,x) in R implies F$. An the only thing that implies a false statement is a false statement. So we must have $(y,x) not in R$.



So in your example you have $(1,2)in Rimplies (2,1)in R$ is true but you don't have $(2,1) in R implies (1,2) in R$ as true.



So it isn't symmetric.



=====



Another way to look at it:



If $A = 1,2,3$



Then we will have 9 statments.



By 1) the nine statements are:



$(1,1)in Rimplies (1,1) in R$



$(1,2) in R implies (2,1) in R$



$(1,3) in R implies (3,1) in R$



$(2,1) in R implies (1,2) in R$



... etc... all nine are needed.



With 2) we also have nine statements:



$(1,1)in Riff (1,1) in R$



$(1,2) in R iff (2,1) in R$



$(1,3) in R iff (3,1) in R$



$(2,1) in R iff (1,2) in R$



...etc....



$(1,2) in R iff (2,1) in R$ and $(2,1) in R iff (1,2)in R$ is redundant.



So aesthetically, using definition 2) is .... inefficient.






share|cite|improve this answer









$endgroup$












  • $begingroup$
    Got it. Thank you, fleablood.
    $endgroup$
    – Rodrigo Sango
    Mar 23 at 16:16






  • 1




    $begingroup$
    "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
    $endgroup$
    – Henning Makholm
    Mar 23 at 18:57










  • $begingroup$
    Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
    $endgroup$
    – fleablood
    Mar 24 at 2:14


















10












$begingroup$

If $A$ is a set and $R$ is a binary relation defined on $A$ (that is, $R$ is a subset of $Atimes A$), then the usual definition of symmetry (as far as $R$ is concerned) is$$(forall xin A)(forall yin A):xmathrel Ryimplies ymathrel Rx.tag1$$And this is equivalent to$$(forall xin A)(forall yin A):xmathrel Ryiff ymathrel Rx.tag2$$So, why do we choose $(1)$ instead of $(2)$ in general? Because, in general (although not in this case) it is easier to verify the assertion $Aimplies B$ than $Aiff B$. And (again, in general), when we choose between two distinct but equivalent definitions, we usually choose the one which is easier to verify that it holds.






share|cite|improve this answer









$endgroup$




















    3












    $begingroup$

    If $xRy implies yRx$ for all $x$ and all $y$, then we can choose $x := tildey$ and $y := tildex$ and get $tildeyRtildex implies tildexRtildey$ or, equivalently, $yRx implies xRy$, which is $impliedby$.



    In conclusion, since the implication should hold for all $x,y$, the equivalence already holds.






    share|cite|improve this answer









    $endgroup$




















      2












      $begingroup$

      Let it be that $R$ is a symmetric relation.



      This according to the first mentioned definition:$$forall xforall y[xRyimplies yRx]tag1$$



      Now let it be that $aRb$.



      Then we are allowed to conclude that $bRa$.



      On the other hand if $bRa$ then also we are conclude that $aRb$.



      So apparantly we have:$$aRbiff bRa$$



      Proved is now that for a symmetric relation $R$ (based on definition $(1)$) we have:$$forall xforall y[xRyiff yRx]tag2$$



      So $(2)$ is a necessary condition for $(1)$.



      Next to that it is obvious that $(2)$ is also a sufficient condition for $(1)$ so actually the statements are equivalent.



      Both can be used as definition then, but in cases like that it is good custom to go for the one with less implications. This for instance decreases the chance on redundant work if we try to prove that a relation is indeed symmetric.






      share|cite|improve this answer











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        4 Answers
        4






        active

        oldest

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        4 Answers
        4






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        7












        $begingroup$

        For all $x$ and all $y$ make the if and only if unnecessary (albeit perfectly acceptable).



        1) $(x,y) in R implies (y,x) in R$ for ALL $x,y in A$



        And the statement 2) $(x,y) in R iff (y,x) in R$ are equivalent statements.



        If 1) is true and $(x,y) not in R$ then although $(x,y)in Rimplies (y,x)in R$ or $F implies (y,x)in R$ is true, it does not tell us any thing about whether or not $(y,x) in R$. However $(y,x) in R implies (x,y) in Y$ tells us that $(y,x) not in R$. Because $(y,x) in R implies (x,y) in R$ means $(y,x) in R implies F$. An the only thing that implies a false statement is a false statement. So we must have $(y,x) not in R$.



        So in your example you have $(1,2)in Rimplies (2,1)in R$ is true but you don't have $(2,1) in R implies (1,2) in R$ as true.



        So it isn't symmetric.



        =====



        Another way to look at it:



        If $A = 1,2,3$



        Then we will have 9 statments.



        By 1) the nine statements are:



        $(1,1)in Rimplies (1,1) in R$



        $(1,2) in R implies (2,1) in R$



        $(1,3) in R implies (3,1) in R$



        $(2,1) in R implies (1,2) in R$



        ... etc... all nine are needed.



        With 2) we also have nine statements:



        $(1,1)in Riff (1,1) in R$



        $(1,2) in R iff (2,1) in R$



        $(1,3) in R iff (3,1) in R$



        $(2,1) in R iff (1,2) in R$



        ...etc....



        $(1,2) in R iff (2,1) in R$ and $(2,1) in R iff (1,2)in R$ is redundant.



        So aesthetically, using definition 2) is .... inefficient.






        share|cite|improve this answer









        $endgroup$












        • $begingroup$
          Got it. Thank you, fleablood.
          $endgroup$
          – Rodrigo Sango
          Mar 23 at 16:16






        • 1




          $begingroup$
          "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
          $endgroup$
          – Henning Makholm
          Mar 23 at 18:57










        • $begingroup$
          Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
          $endgroup$
          – fleablood
          Mar 24 at 2:14















        7












        $begingroup$

        For all $x$ and all $y$ make the if and only if unnecessary (albeit perfectly acceptable).



        1) $(x,y) in R implies (y,x) in R$ for ALL $x,y in A$



        And the statement 2) $(x,y) in R iff (y,x) in R$ are equivalent statements.



        If 1) is true and $(x,y) not in R$ then although $(x,y)in Rimplies (y,x)in R$ or $F implies (y,x)in R$ is true, it does not tell us any thing about whether or not $(y,x) in R$. However $(y,x) in R implies (x,y) in Y$ tells us that $(y,x) not in R$. Because $(y,x) in R implies (x,y) in R$ means $(y,x) in R implies F$. An the only thing that implies a false statement is a false statement. So we must have $(y,x) not in R$.



        So in your example you have $(1,2)in Rimplies (2,1)in R$ is true but you don't have $(2,1) in R implies (1,2) in R$ as true.



        So it isn't symmetric.



        =====



        Another way to look at it:



        If $A = 1,2,3$



        Then we will have 9 statments.



        By 1) the nine statements are:



        $(1,1)in Rimplies (1,1) in R$



        $(1,2) in R implies (2,1) in R$



        $(1,3) in R implies (3,1) in R$



        $(2,1) in R implies (1,2) in R$



        ... etc... all nine are needed.



        With 2) we also have nine statements:



        $(1,1)in Riff (1,1) in R$



        $(1,2) in R iff (2,1) in R$



        $(1,3) in R iff (3,1) in R$



        $(2,1) in R iff (1,2) in R$



        ...etc....



        $(1,2) in R iff (2,1) in R$ and $(2,1) in R iff (1,2)in R$ is redundant.



        So aesthetically, using definition 2) is .... inefficient.






        share|cite|improve this answer









        $endgroup$












        • $begingroup$
          Got it. Thank you, fleablood.
          $endgroup$
          – Rodrigo Sango
          Mar 23 at 16:16






        • 1




          $begingroup$
          "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
          $endgroup$
          – Henning Makholm
          Mar 23 at 18:57










        • $begingroup$
          Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
          $endgroup$
          – fleablood
          Mar 24 at 2:14













        7












        7








        7





        $begingroup$

        For all $x$ and all $y$ make the if and only if unnecessary (albeit perfectly acceptable).



        1) $(x,y) in R implies (y,x) in R$ for ALL $x,y in A$



        And the statement 2) $(x,y) in R iff (y,x) in R$ are equivalent statements.



        If 1) is true and $(x,y) not in R$ then although $(x,y)in Rimplies (y,x)in R$ or $F implies (y,x)in R$ is true, it does not tell us any thing about whether or not $(y,x) in R$. However $(y,x) in R implies (x,y) in Y$ tells us that $(y,x) not in R$. Because $(y,x) in R implies (x,y) in R$ means $(y,x) in R implies F$. An the only thing that implies a false statement is a false statement. So we must have $(y,x) not in R$.



        So in your example you have $(1,2)in Rimplies (2,1)in R$ is true but you don't have $(2,1) in R implies (1,2) in R$ as true.



        So it isn't symmetric.



        =====



        Another way to look at it:



        If $A = 1,2,3$



        Then we will have 9 statments.



        By 1) the nine statements are:



        $(1,1)in Rimplies (1,1) in R$



        $(1,2) in R implies (2,1) in R$



        $(1,3) in R implies (3,1) in R$



        $(2,1) in R implies (1,2) in R$



        ... etc... all nine are needed.



        With 2) we also have nine statements:



        $(1,1)in Riff (1,1) in R$



        $(1,2) in R iff (2,1) in R$



        $(1,3) in R iff (3,1) in R$



        $(2,1) in R iff (1,2) in R$



        ...etc....



        $(1,2) in R iff (2,1) in R$ and $(2,1) in R iff (1,2)in R$ is redundant.



        So aesthetically, using definition 2) is .... inefficient.






        share|cite|improve this answer









        $endgroup$



        For all $x$ and all $y$ make the if and only if unnecessary (albeit perfectly acceptable).



        1) $(x,y) in R implies (y,x) in R$ for ALL $x,y in A$



        And the statement 2) $(x,y) in R iff (y,x) in R$ are equivalent statements.



        If 1) is true and $(x,y) not in R$ then although $(x,y)in Rimplies (y,x)in R$ or $F implies (y,x)in R$ is true, it does not tell us any thing about whether or not $(y,x) in R$. However $(y,x) in R implies (x,y) in Y$ tells us that $(y,x) not in R$. Because $(y,x) in R implies (x,y) in R$ means $(y,x) in R implies F$. An the only thing that implies a false statement is a false statement. So we must have $(y,x) not in R$.



        So in your example you have $(1,2)in Rimplies (2,1)in R$ is true but you don't have $(2,1) in R implies (1,2) in R$ as true.



        So it isn't symmetric.



        =====



        Another way to look at it:



        If $A = 1,2,3$



        Then we will have 9 statments.



        By 1) the nine statements are:



        $(1,1)in Rimplies (1,1) in R$



        $(1,2) in R implies (2,1) in R$



        $(1,3) in R implies (3,1) in R$



        $(2,1) in R implies (1,2) in R$



        ... etc... all nine are needed.



        With 2) we also have nine statements:



        $(1,1)in Riff (1,1) in R$



        $(1,2) in R iff (2,1) in R$



        $(1,3) in R iff (3,1) in R$



        $(2,1) in R iff (1,2) in R$



        ...etc....



        $(1,2) in R iff (2,1) in R$ and $(2,1) in R iff (1,2)in R$ is redundant.



        So aesthetically, using definition 2) is .... inefficient.







        share|cite|improve this answer












        share|cite|improve this answer



        share|cite|improve this answer










        answered Mar 23 at 16:09









        fleabloodfleablood

        1




        1











        • $begingroup$
          Got it. Thank you, fleablood.
          $endgroup$
          – Rodrigo Sango
          Mar 23 at 16:16






        • 1




          $begingroup$
          "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
          $endgroup$
          – Henning Makholm
          Mar 23 at 18:57










        • $begingroup$
          Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
          $endgroup$
          – fleablood
          Mar 24 at 2:14
















        • $begingroup$
          Got it. Thank you, fleablood.
          $endgroup$
          – Rodrigo Sango
          Mar 23 at 16:16






        • 1




          $begingroup$
          "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
          $endgroup$
          – Henning Makholm
          Mar 23 at 18:57










        • $begingroup$
          Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
          $endgroup$
          – fleablood
          Mar 24 at 2:14















        $begingroup$
        Got it. Thank you, fleablood.
        $endgroup$
        – Rodrigo Sango
        Mar 23 at 16:16




        $begingroup$
        Got it. Thank you, fleablood.
        $endgroup$
        – Rodrigo Sango
        Mar 23 at 16:16




        1




        1




        $begingroup$
        "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
        $endgroup$
        – Henning Makholm
        Mar 23 at 18:57




        $begingroup$
        "all nine are needed" ... except the three that are tautologies, of course. But it would be more work and less clear to exclude them than to tolerate them.
        $endgroup$
        – Henning Makholm
        Mar 23 at 18:57












        $begingroup$
        Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
        $endgroup$
        – fleablood
        Mar 24 at 2:14




        $begingroup$
        Well, none of them are needed if you state it as all $(x,y)$ but that includes, for each $(x,y)$, $(y,x)$ as well so... well, why say more than you need to.
        $endgroup$
        – fleablood
        Mar 24 at 2:14











        10












        $begingroup$

        If $A$ is a set and $R$ is a binary relation defined on $A$ (that is, $R$ is a subset of $Atimes A$), then the usual definition of symmetry (as far as $R$ is concerned) is$$(forall xin A)(forall yin A):xmathrel Ryimplies ymathrel Rx.tag1$$And this is equivalent to$$(forall xin A)(forall yin A):xmathrel Ryiff ymathrel Rx.tag2$$So, why do we choose $(1)$ instead of $(2)$ in general? Because, in general (although not in this case) it is easier to verify the assertion $Aimplies B$ than $Aiff B$. And (again, in general), when we choose between two distinct but equivalent definitions, we usually choose the one which is easier to verify that it holds.






        share|cite|improve this answer









        $endgroup$

















          10












          $begingroup$

          If $A$ is a set and $R$ is a binary relation defined on $A$ (that is, $R$ is a subset of $Atimes A$), then the usual definition of symmetry (as far as $R$ is concerned) is$$(forall xin A)(forall yin A):xmathrel Ryimplies ymathrel Rx.tag1$$And this is equivalent to$$(forall xin A)(forall yin A):xmathrel Ryiff ymathrel Rx.tag2$$So, why do we choose $(1)$ instead of $(2)$ in general? Because, in general (although not in this case) it is easier to verify the assertion $Aimplies B$ than $Aiff B$. And (again, in general), when we choose between two distinct but equivalent definitions, we usually choose the one which is easier to verify that it holds.






          share|cite|improve this answer









          $endgroup$















            10












            10








            10





            $begingroup$

            If $A$ is a set and $R$ is a binary relation defined on $A$ (that is, $R$ is a subset of $Atimes A$), then the usual definition of symmetry (as far as $R$ is concerned) is$$(forall xin A)(forall yin A):xmathrel Ryimplies ymathrel Rx.tag1$$And this is equivalent to$$(forall xin A)(forall yin A):xmathrel Ryiff ymathrel Rx.tag2$$So, why do we choose $(1)$ instead of $(2)$ in general? Because, in general (although not in this case) it is easier to verify the assertion $Aimplies B$ than $Aiff B$. And (again, in general), when we choose between two distinct but equivalent definitions, we usually choose the one which is easier to verify that it holds.






            share|cite|improve this answer









            $endgroup$



            If $A$ is a set and $R$ is a binary relation defined on $A$ (that is, $R$ is a subset of $Atimes A$), then the usual definition of symmetry (as far as $R$ is concerned) is$$(forall xin A)(forall yin A):xmathrel Ryimplies ymathrel Rx.tag1$$And this is equivalent to$$(forall xin A)(forall yin A):xmathrel Ryiff ymathrel Rx.tag2$$So, why do we choose $(1)$ instead of $(2)$ in general? Because, in general (although not in this case) it is easier to verify the assertion $Aimplies B$ than $Aiff B$. And (again, in general), when we choose between two distinct but equivalent definitions, we usually choose the one which is easier to verify that it holds.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Mar 23 at 15:25









            José Carlos SantosJosé Carlos Santos

            176k24137246




            176k24137246





















                3












                $begingroup$

                If $xRy implies yRx$ for all $x$ and all $y$, then we can choose $x := tildey$ and $y := tildex$ and get $tildeyRtildex implies tildexRtildey$ or, equivalently, $yRx implies xRy$, which is $impliedby$.



                In conclusion, since the implication should hold for all $x,y$, the equivalence already holds.






                share|cite|improve this answer









                $endgroup$

















                  3












                  $begingroup$

                  If $xRy implies yRx$ for all $x$ and all $y$, then we can choose $x := tildey$ and $y := tildex$ and get $tildeyRtildex implies tildexRtildey$ or, equivalently, $yRx implies xRy$, which is $impliedby$.



                  In conclusion, since the implication should hold for all $x,y$, the equivalence already holds.






                  share|cite|improve this answer









                  $endgroup$















                    3












                    3








                    3





                    $begingroup$

                    If $xRy implies yRx$ for all $x$ and all $y$, then we can choose $x := tildey$ and $y := tildex$ and get $tildeyRtildex implies tildexRtildey$ or, equivalently, $yRx implies xRy$, which is $impliedby$.



                    In conclusion, since the implication should hold for all $x,y$, the equivalence already holds.






                    share|cite|improve this answer









                    $endgroup$



                    If $xRy implies yRx$ for all $x$ and all $y$, then we can choose $x := tildey$ and $y := tildex$ and get $tildeyRtildex implies tildexRtildey$ or, equivalently, $yRx implies xRy$, which is $impliedby$.



                    In conclusion, since the implication should hold for all $x,y$, the equivalence already holds.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Mar 23 at 15:21









                    Viktor GlombikViktor Glombik

                    1,3572628




                    1,3572628





















                        2












                        $begingroup$

                        Let it be that $R$ is a symmetric relation.



                        This according to the first mentioned definition:$$forall xforall y[xRyimplies yRx]tag1$$



                        Now let it be that $aRb$.



                        Then we are allowed to conclude that $bRa$.



                        On the other hand if $bRa$ then also we are conclude that $aRb$.



                        So apparantly we have:$$aRbiff bRa$$



                        Proved is now that for a symmetric relation $R$ (based on definition $(1)$) we have:$$forall xforall y[xRyiff yRx]tag2$$



                        So $(2)$ is a necessary condition for $(1)$.



                        Next to that it is obvious that $(2)$ is also a sufficient condition for $(1)$ so actually the statements are equivalent.



                        Both can be used as definition then, but in cases like that it is good custom to go for the one with less implications. This for instance decreases the chance on redundant work if we try to prove that a relation is indeed symmetric.






                        share|cite|improve this answer











                        $endgroup$

















                          2












                          $begingroup$

                          Let it be that $R$ is a symmetric relation.



                          This according to the first mentioned definition:$$forall xforall y[xRyimplies yRx]tag1$$



                          Now let it be that $aRb$.



                          Then we are allowed to conclude that $bRa$.



                          On the other hand if $bRa$ then also we are conclude that $aRb$.



                          So apparantly we have:$$aRbiff bRa$$



                          Proved is now that for a symmetric relation $R$ (based on definition $(1)$) we have:$$forall xforall y[xRyiff yRx]tag2$$



                          So $(2)$ is a necessary condition for $(1)$.



                          Next to that it is obvious that $(2)$ is also a sufficient condition for $(1)$ so actually the statements are equivalent.



                          Both can be used as definition then, but in cases like that it is good custom to go for the one with less implications. This for instance decreases the chance on redundant work if we try to prove that a relation is indeed symmetric.






                          share|cite|improve this answer











                          $endgroup$















                            2












                            2








                            2





                            $begingroup$

                            Let it be that $R$ is a symmetric relation.



                            This according to the first mentioned definition:$$forall xforall y[xRyimplies yRx]tag1$$



                            Now let it be that $aRb$.



                            Then we are allowed to conclude that $bRa$.



                            On the other hand if $bRa$ then also we are conclude that $aRb$.



                            So apparantly we have:$$aRbiff bRa$$



                            Proved is now that for a symmetric relation $R$ (based on definition $(1)$) we have:$$forall xforall y[xRyiff yRx]tag2$$



                            So $(2)$ is a necessary condition for $(1)$.



                            Next to that it is obvious that $(2)$ is also a sufficient condition for $(1)$ so actually the statements are equivalent.



                            Both can be used as definition then, but in cases like that it is good custom to go for the one with less implications. This for instance decreases the chance on redundant work if we try to prove that a relation is indeed symmetric.






                            share|cite|improve this answer











                            $endgroup$



                            Let it be that $R$ is a symmetric relation.



                            This according to the first mentioned definition:$$forall xforall y[xRyimplies yRx]tag1$$



                            Now let it be that $aRb$.



                            Then we are allowed to conclude that $bRa$.



                            On the other hand if $bRa$ then also we are conclude that $aRb$.



                            So apparantly we have:$$aRbiff bRa$$



                            Proved is now that for a symmetric relation $R$ (based on definition $(1)$) we have:$$forall xforall y[xRyiff yRx]tag2$$



                            So $(2)$ is a necessary condition for $(1)$.



                            Next to that it is obvious that $(2)$ is also a sufficient condition for $(1)$ so actually the statements are equivalent.



                            Both can be used as definition then, but in cases like that it is good custom to go for the one with less implications. This for instance decreases the chance on redundant work if we try to prove that a relation is indeed symmetric.







                            share|cite|improve this answer














                            share|cite|improve this answer



                            share|cite|improve this answer








                            edited Mar 26 at 9:13

























                            answered Mar 23 at 15:30









                            drhabdrhab

                            104k545136




                            104k545136



























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