Why is my proof that $mathbb R$ is disconnected wrong?












-1












$begingroup$


The definition of connectedness in my notes is:
A topological space $X$ is connected if there does not exist a pair of non empty subsets $U$, $V$ such that $Ucap V=emptyset$ and $Ucup V=X$.



However if I have the subsets $(-infty,0]$ and $(0,infty)$ then these are disjoint and cover $mathbb R$ and hence $mathbb R$ is disconnected.



However $mathbb R$ is clearly connected. Where have I gone wrong?










share|cite|improve this question









$endgroup$








  • 5




    $begingroup$
    Open sets. You're missing the point 'open sets'.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:39






  • 1




    $begingroup$
    Yes thank you, that would fix it
    $endgroup$
    – Toby Peterken
    Dec 8 '18 at 11:45










  • $begingroup$
    You're welcome.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:47
















-1












$begingroup$


The definition of connectedness in my notes is:
A topological space $X$ is connected if there does not exist a pair of non empty subsets $U$, $V$ such that $Ucap V=emptyset$ and $Ucup V=X$.



However if I have the subsets $(-infty,0]$ and $(0,infty)$ then these are disjoint and cover $mathbb R$ and hence $mathbb R$ is disconnected.



However $mathbb R$ is clearly connected. Where have I gone wrong?










share|cite|improve this question









$endgroup$








  • 5




    $begingroup$
    Open sets. You're missing the point 'open sets'.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:39






  • 1




    $begingroup$
    Yes thank you, that would fix it
    $endgroup$
    – Toby Peterken
    Dec 8 '18 at 11:45










  • $begingroup$
    You're welcome.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:47














-1












-1








-1





$begingroup$


The definition of connectedness in my notes is:
A topological space $X$ is connected if there does not exist a pair of non empty subsets $U$, $V$ such that $Ucap V=emptyset$ and $Ucup V=X$.



However if I have the subsets $(-infty,0]$ and $(0,infty)$ then these are disjoint and cover $mathbb R$ and hence $mathbb R$ is disconnected.



However $mathbb R$ is clearly connected. Where have I gone wrong?










share|cite|improve this question









$endgroup$




The definition of connectedness in my notes is:
A topological space $X$ is connected if there does not exist a pair of non empty subsets $U$, $V$ such that $Ucap V=emptyset$ and $Ucup V=X$.



However if I have the subsets $(-infty,0]$ and $(0,infty)$ then these are disjoint and cover $mathbb R$ and hence $mathbb R$ is disconnected.



However $mathbb R$ is clearly connected. Where have I gone wrong?







general-topology proof-verification elementary-set-theory connectedness






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 8 '18 at 11:34









Toby PeterkenToby Peterken

1496




1496








  • 5




    $begingroup$
    Open sets. You're missing the point 'open sets'.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:39






  • 1




    $begingroup$
    Yes thank you, that would fix it
    $endgroup$
    – Toby Peterken
    Dec 8 '18 at 11:45










  • $begingroup$
    You're welcome.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:47














  • 5




    $begingroup$
    Open sets. You're missing the point 'open sets'.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:39






  • 1




    $begingroup$
    Yes thank you, that would fix it
    $endgroup$
    – Toby Peterken
    Dec 8 '18 at 11:45










  • $begingroup$
    You're welcome.
    $endgroup$
    – Anik Bhowmick
    Dec 8 '18 at 11:47








5




5




$begingroup$
Open sets. You're missing the point 'open sets'.
$endgroup$
– Anik Bhowmick
Dec 8 '18 at 11:39




$begingroup$
Open sets. You're missing the point 'open sets'.
$endgroup$
– Anik Bhowmick
Dec 8 '18 at 11:39




1




1




$begingroup$
Yes thank you, that would fix it
$endgroup$
– Toby Peterken
Dec 8 '18 at 11:45




$begingroup$
Yes thank you, that would fix it
$endgroup$
– Toby Peterken
Dec 8 '18 at 11:45












$begingroup$
You're welcome.
$endgroup$
– Anik Bhowmick
Dec 8 '18 at 11:47




$begingroup$
You're welcome.
$endgroup$
– Anik Bhowmick
Dec 8 '18 at 11:47










2 Answers
2






active

oldest

votes


















1












$begingroup$

With your definition, every space $X$ with at least two points would be disconnected: just take a point $xin X$ and consider $X={x}cup(Xsetminus{x})$.



The definition requires $U$ and $V$ to be disjoint nonempty open sets such that $Ucup V=X$.



The set $(-infty,0]$ is not open.






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    The subsets that you take are wrong because $(-infty ,0]$ contains a accumulation point of $(0,infty)$.






    share|cite|improve this answer









    $endgroup$













      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%2f3030993%2fwhy-is-my-proof-that-mathbb-r-is-disconnected-wrong%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









      1












      $begingroup$

      With your definition, every space $X$ with at least two points would be disconnected: just take a point $xin X$ and consider $X={x}cup(Xsetminus{x})$.



      The definition requires $U$ and $V$ to be disjoint nonempty open sets such that $Ucup V=X$.



      The set $(-infty,0]$ is not open.






      share|cite|improve this answer









      $endgroup$


















        1












        $begingroup$

        With your definition, every space $X$ with at least two points would be disconnected: just take a point $xin X$ and consider $X={x}cup(Xsetminus{x})$.



        The definition requires $U$ and $V$ to be disjoint nonempty open sets such that $Ucup V=X$.



        The set $(-infty,0]$ is not open.






        share|cite|improve this answer









        $endgroup$
















          1












          1








          1





          $begingroup$

          With your definition, every space $X$ with at least two points would be disconnected: just take a point $xin X$ and consider $X={x}cup(Xsetminus{x})$.



          The definition requires $U$ and $V$ to be disjoint nonempty open sets such that $Ucup V=X$.



          The set $(-infty,0]$ is not open.






          share|cite|improve this answer









          $endgroup$



          With your definition, every space $X$ with at least two points would be disconnected: just take a point $xin X$ and consider $X={x}cup(Xsetminus{x})$.



          The definition requires $U$ and $V$ to be disjoint nonempty open sets such that $Ucup V=X$.



          The set $(-infty,0]$ is not open.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 8 '18 at 11:55









          egregegreg

          181k1485203




          181k1485203























              0












              $begingroup$

              The subsets that you take are wrong because $(-infty ,0]$ contains a accumulation point of $(0,infty)$.






              share|cite|improve this answer









              $endgroup$


















                0












                $begingroup$

                The subsets that you take are wrong because $(-infty ,0]$ contains a accumulation point of $(0,infty)$.






                share|cite|improve this answer









                $endgroup$
















                  0












                  0








                  0





                  $begingroup$

                  The subsets that you take are wrong because $(-infty ,0]$ contains a accumulation point of $(0,infty)$.






                  share|cite|improve this answer









                  $endgroup$



                  The subsets that you take are wrong because $(-infty ,0]$ contains a accumulation point of $(0,infty)$.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered Dec 8 '18 at 11:52









                  Fernando cañizaresFernando cañizares

                  11




                  11






























                      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.




                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function () {
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3030993%2fwhy-is-my-proof-that-mathbb-r-is-disconnected-wrong%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

                      Plaza Victoria

                      Puebla de Zaragoza

                      Musa