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






      share|cite|improve this question















      share|cite|improve this question













      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
          2






          active

          oldest

          votes


















          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





















            Your Answer





            StackExchange.ifUsing("editor", function () {
            return StackExchange.using("mathjaxEditing", function () {
            StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
            StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
            });
            });
            }, "mathjax-editing");

            StackExchange.ready(function() {
            var channelOptions = {
            tags: "".split(" "),
            id: "69"
            };
            initTagRenderer("".split(" "), "".split(" "), channelOptions);

            StackExchange.using("externalEditor", function() {
            // Have to fire editor after snippets, if snippets enabled
            if (StackExchange.settings.snippets.snippetsEnabled) {
            StackExchange.using("snippets", function() {
            createEditor();
            });
            }
            else {
            createEditor();
            }
            });

            function createEditor() {
            StackExchange.prepareEditor({
            heartbeatType: 'answer',
            autoActivateHeartbeat: false,
            convertImagesToLinks: true,
            noModals: true,
            showLowRepImageUploadWarning: true,
            reputationToPostImages: 10,
            bindNavPrevention: true,
            postfix: "",
            imageUploader: {
            brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
            contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
            allowUrls: true
            },
            noCode: true, onDemand: true,
            discardSelector: ".discard-answer"
            ,immediatelyShowMarkdownHelp:true
            });


            }
            });














            draft saved

            draft discarded


















            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f736457%2fsuppose-y-subset-x-and-x-y-are-connected-and-a-b-form-separation-for-x-y%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown
























            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            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






























                    draft saved

                    draft discarded




















































                    Thanks for contributing an answer to Mathematics Stack Exchange!


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    Use MathJax to format equations. MathJax reference.


                    To learn more, see our tips on writing great answers.





                    Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


                    Please pay close attention to the following guidance:


                    • Please be sure to answer the question. Provide details and share your research!

                    But avoid



                    • Asking for help, clarification, or responding to other answers.

                    • Making statements based on opinion; back them up with references or personal experience.


                    To learn more, see our tips on writing great answers.




                    draft saved


                    draft discarded














                    StackExchange.ready(
                    function () {
                    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f736457%2fsuppose-y-subset-x-and-x-y-are-connected-and-a-b-form-separation-for-x-y%23new-answer', 'question_page');
                    }
                    );

                    Post as a guest















                    Required, but never shown





















































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown

































                    Required, but never shown














                    Required, but never shown












                    Required, but never shown







                    Required, but never shown







                    Popular posts from this blog

                    1300-talet

                    1300-talet

                    Display a custom attribute below product name in the front-end Magento 1.9.3.8