Suppose $Ysubset X$ and $X,Y$ are connected and $A,B$ form separation for $X-Y$ then, prove that $Ycup A$ and...












3














Question is :



Suppose $Ysubset X$ and $X,Y$ are connected and $A,B$ form separation for $X-Y$ then, Prove that $Ycup A$ and $Ycup B$ are connected.



What i have tried is :



Suppose $Ycup A$ has separation say $Ccup D$ then all i could see is that



Either $Ysubset C$ or $Ysubset D$ as $Y$ is connected and $
Ccup D$ is separation for a subset that contains $Y$.



Without loss of generality we could assume $Ysubset C$



As $Ycup A=Ccup D$ and $Ysubset C$ i can say $Dsubset A$ (I do not know how does this help)



I have $X-Y=Acup B$ with $Acap B=emptyset$



I have not used connectedness of $X$ till now, So i thought of using that and end up with following :



$X-Y=Acup BRightarrow X=Ycup Acup B=(Ycup A)cup (Ycup B)$



I do not know what to conclude from this...



I would be thankful if some one can help me by giving some "hints"



Thank you










share|cite|improve this question





























    3














    Question is :



    Suppose $Ysubset X$ and $X,Y$ are connected and $A,B$ form separation for $X-Y$ then, Prove that $Ycup A$ and $Ycup B$ are connected.



    What i have tried is :



    Suppose $Ycup A$ has separation say $Ccup D$ then all i could see is that



    Either $Ysubset C$ or $Ysubset D$ as $Y$ is connected and $
    Ccup D$ is separation for a subset that contains $Y$.



    Without loss of generality we could assume $Ysubset C$



    As $Ycup A=Ccup D$ and $Ysubset C$ i can say $Dsubset A$ (I do not know how does this help)



    I have $X-Y=Acup B$ with $Acap B=emptyset$



    I have not used connectedness of $X$ till now, So i thought of using that and end up with following :



    $X-Y=Acup BRightarrow X=Ycup Acup B=(Ycup A)cup (Ycup B)$



    I do not know what to conclude from this...



    I would be thankful if some one can help me by giving some "hints"



    Thank you










    share|cite|improve this question



























      3












      3








      3


      1





      Question is :



      Suppose $Ysubset X$ and $X,Y$ are connected and $A,B$ form separation for $X-Y$ then, Prove that $Ycup A$ and $Ycup B$ are connected.



      What i have tried is :



      Suppose $Ycup A$ has separation say $Ccup D$ then all i could see is that



      Either $Ysubset C$ or $Ysubset D$ as $Y$ is connected and $
      Ccup D$ is separation for a subset that contains $Y$.



      Without loss of generality we could assume $Ysubset C$



      As $Ycup A=Ccup D$ and $Ysubset C$ i can say $Dsubset A$ (I do not know how does this help)



      I have $X-Y=Acup B$ with $Acap B=emptyset$



      I have not used connectedness of $X$ till now, So i thought of using that and end up with following :



      $X-Y=Acup BRightarrow X=Ycup Acup B=(Ycup A)cup (Ycup B)$



      I do not know what to conclude from this...



      I would be thankful if some one can help me by giving some "hints"



      Thank you










      share|cite|improve this question















      Question is :



      Suppose $Ysubset X$ and $X,Y$ are connected and $A,B$ form separation for $X-Y$ then, Prove that $Ycup A$ and $Ycup B$ are connected.



      What i have tried is :



      Suppose $Ycup A$ has separation say $Ccup D$ then all i could see is that



      Either $Ysubset C$ or $Ysubset D$ as $Y$ is connected and $
      Ccup D$ is separation for a subset that contains $Y$.



      Without loss of generality we could assume $Ysubset C$



      As $Ycup A=Ccup D$ and $Ysubset C$ i can say $Dsubset A$ (I do not know how does this help)



      I have $X-Y=Acup B$ with $Acap B=emptyset$



      I have not used connectedness of $X$ till now, So i thought of using that and end up with following :



      $X-Y=Acup BRightarrow X=Ycup Acup B=(Ycup A)cup (Ycup B)$



      I do not know what to conclude from this...



      I would be thankful if some one can help me by giving some "hints"



      Thank you







      general-topology connectedness






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      share|cite|improve this question




      share|cite|improve this question








      edited Jun 27 '16 at 10:47









      Irregular User

      2,86251843




      2,86251843










      asked Apr 2 '14 at 11:34







      user87543





























          2 Answers
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          7














          As $A,B$ form a separation for $X-Y$ and $X$ is connected, $Y$ must be non-empty.

          You suppose that $Ycup A$ has a separation $Ccup D$. Then $Y$, being connected, is a subset of $C$



          Now $A$ is clopen in $X-Y$, that means there are sets $U$ open and $K$ closed, such that $U-Y=K-Y=A$. You know that $Dsubseteq A$ is clopen in $Ycup A$. Can you show that $D$ is open in $U$ and closed in $K$?






          share|cite|improve this answer























          • Second paragraph is confusing me... could you please edit it a bit...
            – user87543
            Apr 3 '14 at 4:40










          • @PraphullaKoushik: Which sentence confuses you?
            – Stefan Hamcke
            Apr 3 '14 at 14:14






          • 1




            Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
            – Stefan Hamcke
            Apr 3 '14 at 14:54






          • 2




            @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
            – Stefan Hamcke
            Aug 21 '14 at 16:31






          • 1




            @MathsLover: Yes, that argument is right :)
            – Stefan Hamcke
            Aug 21 '14 at 19:12



















          0














          Continuing with your argument, We can prove that $B cup C$ and $D$ form the partition of $X$.(Assuming your conclusion that $D subset A$).



          We have $X = (B cup C) cup D$.



          $C$ and $D$ form separation of $Y cup A$. So, using theorem (23.1), no limit point of $C$ is in $D$. Similarly, no limit point of $B$ is in $D$ since $D subset A$ and $A$ and $B$ form separation of $X-Y$. From this, we can easily say that no limit point of $B cup C$ is in $D$ i.e. Every limit point of $B cup C$ wrt $X$ is in $B cup C$. So, $B cup C$ is closed in $X$ and $D$ is open in $X$.



          Now, use a similar argument that, no limit point of $D$ is in $B cup C$ implying that $D$ is closed in $X$ and $Bcup C$ is open in X. Hence, $B cup C$ and $D$ are $textit{clopen}$ in $X$ and form partition of $X$.






          share|cite|improve this answer





















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            2 Answers
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            2 Answers
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            7














            As $A,B$ form a separation for $X-Y$ and $X$ is connected, $Y$ must be non-empty.

            You suppose that $Ycup A$ has a separation $Ccup D$. Then $Y$, being connected, is a subset of $C$



            Now $A$ is clopen in $X-Y$, that means there are sets $U$ open and $K$ closed, such that $U-Y=K-Y=A$. You know that $Dsubseteq A$ is clopen in $Ycup A$. Can you show that $D$ is open in $U$ and closed in $K$?






            share|cite|improve this answer























            • Second paragraph is confusing me... could you please edit it a bit...
              – user87543
              Apr 3 '14 at 4:40










            • @PraphullaKoushik: Which sentence confuses you?
              – Stefan Hamcke
              Apr 3 '14 at 14:14






            • 1




              Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
              – Stefan Hamcke
              Apr 3 '14 at 14:54






            • 2




              @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
              – Stefan Hamcke
              Aug 21 '14 at 16:31






            • 1




              @MathsLover: Yes, that argument is right :)
              – Stefan Hamcke
              Aug 21 '14 at 19:12
















            7














            As $A,B$ form a separation for $X-Y$ and $X$ is connected, $Y$ must be non-empty.

            You suppose that $Ycup A$ has a separation $Ccup D$. Then $Y$, being connected, is a subset of $C$



            Now $A$ is clopen in $X-Y$, that means there are sets $U$ open and $K$ closed, such that $U-Y=K-Y=A$. You know that $Dsubseteq A$ is clopen in $Ycup A$. Can you show that $D$ is open in $U$ and closed in $K$?






            share|cite|improve this answer























            • Second paragraph is confusing me... could you please edit it a bit...
              – user87543
              Apr 3 '14 at 4:40










            • @PraphullaKoushik: Which sentence confuses you?
              – Stefan Hamcke
              Apr 3 '14 at 14:14






            • 1




              Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
              – Stefan Hamcke
              Apr 3 '14 at 14:54






            • 2




              @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
              – Stefan Hamcke
              Aug 21 '14 at 16:31






            • 1




              @MathsLover: Yes, that argument is right :)
              – Stefan Hamcke
              Aug 21 '14 at 19:12














            7












            7








            7






            As $A,B$ form a separation for $X-Y$ and $X$ is connected, $Y$ must be non-empty.

            You suppose that $Ycup A$ has a separation $Ccup D$. Then $Y$, being connected, is a subset of $C$



            Now $A$ is clopen in $X-Y$, that means there are sets $U$ open and $K$ closed, such that $U-Y=K-Y=A$. You know that $Dsubseteq A$ is clopen in $Ycup A$. Can you show that $D$ is open in $U$ and closed in $K$?






            share|cite|improve this answer














            As $A,B$ form a separation for $X-Y$ and $X$ is connected, $Y$ must be non-empty.

            You suppose that $Ycup A$ has a separation $Ccup D$. Then $Y$, being connected, is a subset of $C$



            Now $A$ is clopen in $X-Y$, that means there are sets $U$ open and $K$ closed, such that $U-Y=K-Y=A$. You know that $Dsubseteq A$ is clopen in $Ycup A$. Can you show that $D$ is open in $U$ and closed in $K$?







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Jun 29 '14 at 20:16

























            answered Apr 2 '14 at 14:09









            Stefan HamckeStefan Hamcke

            21.6k42877




            21.6k42877












            • Second paragraph is confusing me... could you please edit it a bit...
              – user87543
              Apr 3 '14 at 4:40










            • @PraphullaKoushik: Which sentence confuses you?
              – Stefan Hamcke
              Apr 3 '14 at 14:14






            • 1




              Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
              – Stefan Hamcke
              Apr 3 '14 at 14:54






            • 2




              @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
              – Stefan Hamcke
              Aug 21 '14 at 16:31






            • 1




              @MathsLover: Yes, that argument is right :)
              – Stefan Hamcke
              Aug 21 '14 at 19:12


















            • Second paragraph is confusing me... could you please edit it a bit...
              – user87543
              Apr 3 '14 at 4:40










            • @PraphullaKoushik: Which sentence confuses you?
              – Stefan Hamcke
              Apr 3 '14 at 14:14






            • 1




              Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
              – Stefan Hamcke
              Apr 3 '14 at 14:54






            • 2




              @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
              – Stefan Hamcke
              Aug 21 '14 at 16:31






            • 1




              @MathsLover: Yes, that argument is right :)
              – Stefan Hamcke
              Aug 21 '14 at 19:12
















            Second paragraph is confusing me... could you please edit it a bit...
            – user87543
            Apr 3 '14 at 4:40




            Second paragraph is confusing me... could you please edit it a bit...
            – user87543
            Apr 3 '14 at 4:40












            @PraphullaKoushik: Which sentence confuses you?
            – Stefan Hamcke
            Apr 3 '14 at 14:14




            @PraphullaKoushik: Which sentence confuses you?
            – Stefan Hamcke
            Apr 3 '14 at 14:14




            1




            1




            Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
            – Stefan Hamcke
            Apr 3 '14 at 14:54




            Well, I should say I had already solved this problem some time ago, then forgot the proof so I had to solve it again, but it wasn"t entirely new to me. When I deal with such a problem I usually draw a picture and draw into it the sets that I can think of, in order to see how they relate. Here it was a large disk $X$, a smaller disk $Y$ within $X$, and two lines from the outer circle to the inner circle which you can think of separating $X-Y$ into $A$ and $B$. Then I separated $Acup Y$ into $D$ and $C$ where $D$ was just a subset of $A$ ... (to be continued)
            – Stefan Hamcke
            Apr 3 '14 at 14:54




            2




            2




            @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
            – Stefan Hamcke
            Aug 21 '14 at 16:31




            @MathsLover: Actually, I don't define anything here. $A$ is clopen in $X-Y$ if it's both open and closed in $X-Y$. And this implies the existence of the sets $U$ and $K$.
            – Stefan Hamcke
            Aug 21 '14 at 16:31




            1




            1




            @MathsLover: Yes, that argument is right :)
            – Stefan Hamcke
            Aug 21 '14 at 19:12




            @MathsLover: Yes, that argument is right :)
            – Stefan Hamcke
            Aug 21 '14 at 19:12











            0














            Continuing with your argument, We can prove that $B cup C$ and $D$ form the partition of $X$.(Assuming your conclusion that $D subset A$).



            We have $X = (B cup C) cup D$.



            $C$ and $D$ form separation of $Y cup A$. So, using theorem (23.1), no limit point of $C$ is in $D$. Similarly, no limit point of $B$ is in $D$ since $D subset A$ and $A$ and $B$ form separation of $X-Y$. From this, we can easily say that no limit point of $B cup C$ is in $D$ i.e. Every limit point of $B cup C$ wrt $X$ is in $B cup C$. So, $B cup C$ is closed in $X$ and $D$ is open in $X$.



            Now, use a similar argument that, no limit point of $D$ is in $B cup C$ implying that $D$ is closed in $X$ and $Bcup C$ is open in X. Hence, $B cup C$ and $D$ are $textit{clopen}$ in $X$ and form partition of $X$.






            share|cite|improve this answer


























              0














              Continuing with your argument, We can prove that $B cup C$ and $D$ form the partition of $X$.(Assuming your conclusion that $D subset A$).



              We have $X = (B cup C) cup D$.



              $C$ and $D$ form separation of $Y cup A$. So, using theorem (23.1), no limit point of $C$ is in $D$. Similarly, no limit point of $B$ is in $D$ since $D subset A$ and $A$ and $B$ form separation of $X-Y$. From this, we can easily say that no limit point of $B cup C$ is in $D$ i.e. Every limit point of $B cup C$ wrt $X$ is in $B cup C$. So, $B cup C$ is closed in $X$ and $D$ is open in $X$.



              Now, use a similar argument that, no limit point of $D$ is in $B cup C$ implying that $D$ is closed in $X$ and $Bcup C$ is open in X. Hence, $B cup C$ and $D$ are $textit{clopen}$ in $X$ and form partition of $X$.






              share|cite|improve this answer
























                0












                0








                0






                Continuing with your argument, We can prove that $B cup C$ and $D$ form the partition of $X$.(Assuming your conclusion that $D subset A$).



                We have $X = (B cup C) cup D$.



                $C$ and $D$ form separation of $Y cup A$. So, using theorem (23.1), no limit point of $C$ is in $D$. Similarly, no limit point of $B$ is in $D$ since $D subset A$ and $A$ and $B$ form separation of $X-Y$. From this, we can easily say that no limit point of $B cup C$ is in $D$ i.e. Every limit point of $B cup C$ wrt $X$ is in $B cup C$. So, $B cup C$ is closed in $X$ and $D$ is open in $X$.



                Now, use a similar argument that, no limit point of $D$ is in $B cup C$ implying that $D$ is closed in $X$ and $Bcup C$ is open in X. Hence, $B cup C$ and $D$ are $textit{clopen}$ in $X$ and form partition of $X$.






                share|cite|improve this answer












                Continuing with your argument, We can prove that $B cup C$ and $D$ form the partition of $X$.(Assuming your conclusion that $D subset A$).



                We have $X = (B cup C) cup D$.



                $C$ and $D$ form separation of $Y cup A$. So, using theorem (23.1), no limit point of $C$ is in $D$. Similarly, no limit point of $B$ is in $D$ since $D subset A$ and $A$ and $B$ form separation of $X-Y$. From this, we can easily say that no limit point of $B cup C$ is in $D$ i.e. Every limit point of $B cup C$ wrt $X$ is in $B cup C$. So, $B cup C$ is closed in $X$ and $D$ is open in $X$.



                Now, use a similar argument that, no limit point of $D$ is in $B cup C$ implying that $D$ is closed in $X$ and $Bcup C$ is open in X. Hence, $B cup C$ and $D$ are $textit{clopen}$ in $X$ and form partition of $X$.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 4 at 8:46









                MUHMUH

                395216




                395216






























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