Necessary condition on homology group for a set to be contractible












4












$begingroup$


We call a topological space is contractible iff it is homotopic to a point. Since homology group is homotopy invariant, we can see that under any abelian group as coefficients set, a topological space $(X, tau)$ has $H_1(X) = 0$ if $X$ is contractible.



Now, can we find a necessary condition on the homology group of $X$ that can imply X is contractible using some abelian groups as coefficients? The reason why I want to focus on $H_1(X)$ is because, if a space is not contractible, then there will be a 1-chain that can not be deformed to a point while a 2-face can always be deformed to a point.



I noticed that when using $mathbb{Q}$ as the coefficients, "$H_1(X) = 0$" can not imply $X$ is contractible. The conterexample is the projective plane of order 2, $mathbb{P}^2$. When using $mathbb{Z}$ as coefficients, then for any $n >= 2$, $S^n$ (the n-sphere) has homology 1-group equal to $0$ but all of them are not contractible.



Could anyone find an abelian group $G$ such that I can conclude "using $G$ as the coefficients set, $H_1(X) = 0$ implies $X$ is contractible"?
Furthermore, if no matter what coefficients set I use, $H_1(X)$ is always $0$, can I conclude that $X$ is contractible?










share|cite|improve this question









$endgroup$

















    4












    $begingroup$


    We call a topological space is contractible iff it is homotopic to a point. Since homology group is homotopy invariant, we can see that under any abelian group as coefficients set, a topological space $(X, tau)$ has $H_1(X) = 0$ if $X$ is contractible.



    Now, can we find a necessary condition on the homology group of $X$ that can imply X is contractible using some abelian groups as coefficients? The reason why I want to focus on $H_1(X)$ is because, if a space is not contractible, then there will be a 1-chain that can not be deformed to a point while a 2-face can always be deformed to a point.



    I noticed that when using $mathbb{Q}$ as the coefficients, "$H_1(X) = 0$" can not imply $X$ is contractible. The conterexample is the projective plane of order 2, $mathbb{P}^2$. When using $mathbb{Z}$ as coefficients, then for any $n >= 2$, $S^n$ (the n-sphere) has homology 1-group equal to $0$ but all of them are not contractible.



    Could anyone find an abelian group $G$ such that I can conclude "using $G$ as the coefficients set, $H_1(X) = 0$ implies $X$ is contractible"?
    Furthermore, if no matter what coefficients set I use, $H_1(X)$ is always $0$, can I conclude that $X$ is contractible?










    share|cite|improve this question









    $endgroup$















      4












      4








      4





      $begingroup$


      We call a topological space is contractible iff it is homotopic to a point. Since homology group is homotopy invariant, we can see that under any abelian group as coefficients set, a topological space $(X, tau)$ has $H_1(X) = 0$ if $X$ is contractible.



      Now, can we find a necessary condition on the homology group of $X$ that can imply X is contractible using some abelian groups as coefficients? The reason why I want to focus on $H_1(X)$ is because, if a space is not contractible, then there will be a 1-chain that can not be deformed to a point while a 2-face can always be deformed to a point.



      I noticed that when using $mathbb{Q}$ as the coefficients, "$H_1(X) = 0$" can not imply $X$ is contractible. The conterexample is the projective plane of order 2, $mathbb{P}^2$. When using $mathbb{Z}$ as coefficients, then for any $n >= 2$, $S^n$ (the n-sphere) has homology 1-group equal to $0$ but all of them are not contractible.



      Could anyone find an abelian group $G$ such that I can conclude "using $G$ as the coefficients set, $H_1(X) = 0$ implies $X$ is contractible"?
      Furthermore, if no matter what coefficients set I use, $H_1(X)$ is always $0$, can I conclude that $X$ is contractible?










      share|cite|improve this question









      $endgroup$




      We call a topological space is contractible iff it is homotopic to a point. Since homology group is homotopy invariant, we can see that under any abelian group as coefficients set, a topological space $(X, tau)$ has $H_1(X) = 0$ if $X$ is contractible.



      Now, can we find a necessary condition on the homology group of $X$ that can imply X is contractible using some abelian groups as coefficients? The reason why I want to focus on $H_1(X)$ is because, if a space is not contractible, then there will be a 1-chain that can not be deformed to a point while a 2-face can always be deformed to a point.



      I noticed that when using $mathbb{Q}$ as the coefficients, "$H_1(X) = 0$" can not imply $X$ is contractible. The conterexample is the projective plane of order 2, $mathbb{P}^2$. When using $mathbb{Z}$ as coefficients, then for any $n >= 2$, $S^n$ (the n-sphere) has homology 1-group equal to $0$ but all of them are not contractible.



      Could anyone find an abelian group $G$ such that I can conclude "using $G$ as the coefficients set, $H_1(X) = 0$ implies $X$ is contractible"?
      Furthermore, if no matter what coefficients set I use, $H_1(X)$ is always $0$, can I conclude that $X$ is contractible?







      algebraic-topology simplicial-complex






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked 4 hours ago









      Sanae KochiyaSanae Kochiya

      676




      676






















          3 Answers
          3






          active

          oldest

          votes


















          2












          $begingroup$

          The first homology group is far from enough to detect contractibility, since spaces can have non-vanishing higher homology groups.



          It's not even enough to have $H_n(X;G)$ vanish for every $n$ and $G$. For one thing there are spaces which are weakly contractible (i.e. all their homotopy vanish and hence their homology as well) but which are not contractible, like the Warsaw Circle.



          By Whitehead's Theorem a weakly contractible space which is not contractible cannot have the homotopy type of a CW complex, so we can ask if vanishing homology is enough to conclude that a CW complex is contractible. This still is not enough, because we can take the $2$-skeleton $S$ of the Poincare homology $3$-sphere, which is a finite $2$-dimensional CW complex whose homology groups vanish with any coefficients, but $pi_1(S)$ has order $120$ so it's not contractible.



          However there is an affirmative answer to your question that involves the fundamental group. If $X$ is a CW complex such that $pi_1(X) = 0$ and $H_n(X;mathbb{Z})=0$ for $n > 1$, then it follows by Whitehead's Theorem and the Hurewicz Theorem that $X$ is contractible.






          share|cite|improve this answer









          $endgroup$





















            3












            $begingroup$

            A counterexample is the sphere $S^2$, whose first homology group will vanish for any coefficients, but which is not contractible (because its second homology group doesn't vanish).






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              Thank you for your response. Do you mind direct me to the proof of your statement?
              $endgroup$
              – Sanae Kochiya
              3 hours ago










            • $begingroup$
              For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
              $endgroup$
              – hunter
              2 hours ago



















            0












            $begingroup$

            This is a very good question because this is exactly what early algebraic topologists cared about! The general case is no; there are no conditions on homology that are sufficient to say a space is contractible. The double comb space (https://topospaces.subwiki.org/wiki/Double_comb_space) is a space whose homology (and homotopy) groups are all trivial with coefficients in any group. It also is not contractible meaning it is not homotopy equivalent to a point.



            But when you have a great question, a counterexample should not dissuade you. Can we put restrictions on a space so that trivial homology (with coefficients in integers) implies it is contractible? The answer is yes. If we restrict to CW complexes, you can prove that any map that induces an isomorphism on all homotopy groups must be a homotopy equivalence. This is called Whitehead's theorem. One of its corollaries is that between simply connected CW complexes, any map that induces isomorphisms on homology groups is a homotopy equivalence. This means that a simply connected CW complex with trivial homology is contractible since the map to a point induces isomorphisms on homology.






            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%2f3168982%2fnecessary-condition-on-homology-group-for-a-set-to-be-contractible%23new-answer', 'question_page');
              }
              );

              Post as a guest















              Required, but never shown

























              3 Answers
              3






              active

              oldest

              votes








              3 Answers
              3






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              2












              $begingroup$

              The first homology group is far from enough to detect contractibility, since spaces can have non-vanishing higher homology groups.



              It's not even enough to have $H_n(X;G)$ vanish for every $n$ and $G$. For one thing there are spaces which are weakly contractible (i.e. all their homotopy vanish and hence their homology as well) but which are not contractible, like the Warsaw Circle.



              By Whitehead's Theorem a weakly contractible space which is not contractible cannot have the homotopy type of a CW complex, so we can ask if vanishing homology is enough to conclude that a CW complex is contractible. This still is not enough, because we can take the $2$-skeleton $S$ of the Poincare homology $3$-sphere, which is a finite $2$-dimensional CW complex whose homology groups vanish with any coefficients, but $pi_1(S)$ has order $120$ so it's not contractible.



              However there is an affirmative answer to your question that involves the fundamental group. If $X$ is a CW complex such that $pi_1(X) = 0$ and $H_n(X;mathbb{Z})=0$ for $n > 1$, then it follows by Whitehead's Theorem and the Hurewicz Theorem that $X$ is contractible.






              share|cite|improve this answer









              $endgroup$


















                2












                $begingroup$

                The first homology group is far from enough to detect contractibility, since spaces can have non-vanishing higher homology groups.



                It's not even enough to have $H_n(X;G)$ vanish for every $n$ and $G$. For one thing there are spaces which are weakly contractible (i.e. all their homotopy vanish and hence their homology as well) but which are not contractible, like the Warsaw Circle.



                By Whitehead's Theorem a weakly contractible space which is not contractible cannot have the homotopy type of a CW complex, so we can ask if vanishing homology is enough to conclude that a CW complex is contractible. This still is not enough, because we can take the $2$-skeleton $S$ of the Poincare homology $3$-sphere, which is a finite $2$-dimensional CW complex whose homology groups vanish with any coefficients, but $pi_1(S)$ has order $120$ so it's not contractible.



                However there is an affirmative answer to your question that involves the fundamental group. If $X$ is a CW complex such that $pi_1(X) = 0$ and $H_n(X;mathbb{Z})=0$ for $n > 1$, then it follows by Whitehead's Theorem and the Hurewicz Theorem that $X$ is contractible.






                share|cite|improve this answer









                $endgroup$
















                  2












                  2








                  2





                  $begingroup$

                  The first homology group is far from enough to detect contractibility, since spaces can have non-vanishing higher homology groups.



                  It's not even enough to have $H_n(X;G)$ vanish for every $n$ and $G$. For one thing there are spaces which are weakly contractible (i.e. all their homotopy vanish and hence their homology as well) but which are not contractible, like the Warsaw Circle.



                  By Whitehead's Theorem a weakly contractible space which is not contractible cannot have the homotopy type of a CW complex, so we can ask if vanishing homology is enough to conclude that a CW complex is contractible. This still is not enough, because we can take the $2$-skeleton $S$ of the Poincare homology $3$-sphere, which is a finite $2$-dimensional CW complex whose homology groups vanish with any coefficients, but $pi_1(S)$ has order $120$ so it's not contractible.



                  However there is an affirmative answer to your question that involves the fundamental group. If $X$ is a CW complex such that $pi_1(X) = 0$ and $H_n(X;mathbb{Z})=0$ for $n > 1$, then it follows by Whitehead's Theorem and the Hurewicz Theorem that $X$ is contractible.






                  share|cite|improve this answer









                  $endgroup$



                  The first homology group is far from enough to detect contractibility, since spaces can have non-vanishing higher homology groups.



                  It's not even enough to have $H_n(X;G)$ vanish for every $n$ and $G$. For one thing there are spaces which are weakly contractible (i.e. all their homotopy vanish and hence their homology as well) but which are not contractible, like the Warsaw Circle.



                  By Whitehead's Theorem a weakly contractible space which is not contractible cannot have the homotopy type of a CW complex, so we can ask if vanishing homology is enough to conclude that a CW complex is contractible. This still is not enough, because we can take the $2$-skeleton $S$ of the Poincare homology $3$-sphere, which is a finite $2$-dimensional CW complex whose homology groups vanish with any coefficients, but $pi_1(S)$ has order $120$ so it's not contractible.



                  However there is an affirmative answer to your question that involves the fundamental group. If $X$ is a CW complex such that $pi_1(X) = 0$ and $H_n(X;mathbb{Z})=0$ for $n > 1$, then it follows by Whitehead's Theorem and the Hurewicz Theorem that $X$ is contractible.







                  share|cite|improve this answer












                  share|cite|improve this answer



                  share|cite|improve this answer










                  answered 3 hours ago









                  WilliamWilliam

                  2,9451225




                  2,9451225























                      3












                      $begingroup$

                      A counterexample is the sphere $S^2$, whose first homology group will vanish for any coefficients, but which is not contractible (because its second homology group doesn't vanish).






                      share|cite|improve this answer









                      $endgroup$













                      • $begingroup$
                        Thank you for your response. Do you mind direct me to the proof of your statement?
                        $endgroup$
                        – Sanae Kochiya
                        3 hours ago










                      • $begingroup$
                        For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                        $endgroup$
                        – hunter
                        2 hours ago
















                      3












                      $begingroup$

                      A counterexample is the sphere $S^2$, whose first homology group will vanish for any coefficients, but which is not contractible (because its second homology group doesn't vanish).






                      share|cite|improve this answer









                      $endgroup$













                      • $begingroup$
                        Thank you for your response. Do you mind direct me to the proof of your statement?
                        $endgroup$
                        – Sanae Kochiya
                        3 hours ago










                      • $begingroup$
                        For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                        $endgroup$
                        – hunter
                        2 hours ago














                      3












                      3








                      3





                      $begingroup$

                      A counterexample is the sphere $S^2$, whose first homology group will vanish for any coefficients, but which is not contractible (because its second homology group doesn't vanish).






                      share|cite|improve this answer









                      $endgroup$



                      A counterexample is the sphere $S^2$, whose first homology group will vanish for any coefficients, but which is not contractible (because its second homology group doesn't vanish).







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered 3 hours ago









                      hunterhunter

                      15.4k32640




                      15.4k32640












                      • $begingroup$
                        Thank you for your response. Do you mind direct me to the proof of your statement?
                        $endgroup$
                        – Sanae Kochiya
                        3 hours ago










                      • $begingroup$
                        For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                        $endgroup$
                        – hunter
                        2 hours ago


















                      • $begingroup$
                        Thank you for your response. Do you mind direct me to the proof of your statement?
                        $endgroup$
                        – Sanae Kochiya
                        3 hours ago










                      • $begingroup$
                        For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                        $endgroup$
                        – hunter
                        2 hours ago
















                      $begingroup$
                      Thank you for your response. Do you mind direct me to the proof of your statement?
                      $endgroup$
                      – Sanae Kochiya
                      3 hours ago




                      $begingroup$
                      Thank you for your response. Do you mind direct me to the proof of your statement?
                      $endgroup$
                      – Sanae Kochiya
                      3 hours ago












                      $begingroup$
                      For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                      $endgroup$
                      – hunter
                      2 hours ago




                      $begingroup$
                      For any abelian group of coefficients $A$, we have $H_1(S^2, A) = H_1(S^2, mathbb{Z}) otimes A$, e.g. by the universal coefficient theorem (since there's no torsion in the other homology groups).
                      $endgroup$
                      – hunter
                      2 hours ago











                      0












                      $begingroup$

                      This is a very good question because this is exactly what early algebraic topologists cared about! The general case is no; there are no conditions on homology that are sufficient to say a space is contractible. The double comb space (https://topospaces.subwiki.org/wiki/Double_comb_space) is a space whose homology (and homotopy) groups are all trivial with coefficients in any group. It also is not contractible meaning it is not homotopy equivalent to a point.



                      But when you have a great question, a counterexample should not dissuade you. Can we put restrictions on a space so that trivial homology (with coefficients in integers) implies it is contractible? The answer is yes. If we restrict to CW complexes, you can prove that any map that induces an isomorphism on all homotopy groups must be a homotopy equivalence. This is called Whitehead's theorem. One of its corollaries is that between simply connected CW complexes, any map that induces isomorphisms on homology groups is a homotopy equivalence. This means that a simply connected CW complex with trivial homology is contractible since the map to a point induces isomorphisms on homology.






                      share|cite|improve this answer









                      $endgroup$


















                        0












                        $begingroup$

                        This is a very good question because this is exactly what early algebraic topologists cared about! The general case is no; there are no conditions on homology that are sufficient to say a space is contractible. The double comb space (https://topospaces.subwiki.org/wiki/Double_comb_space) is a space whose homology (and homotopy) groups are all trivial with coefficients in any group. It also is not contractible meaning it is not homotopy equivalent to a point.



                        But when you have a great question, a counterexample should not dissuade you. Can we put restrictions on a space so that trivial homology (with coefficients in integers) implies it is contractible? The answer is yes. If we restrict to CW complexes, you can prove that any map that induces an isomorphism on all homotopy groups must be a homotopy equivalence. This is called Whitehead's theorem. One of its corollaries is that between simply connected CW complexes, any map that induces isomorphisms on homology groups is a homotopy equivalence. This means that a simply connected CW complex with trivial homology is contractible since the map to a point induces isomorphisms on homology.






                        share|cite|improve this answer









                        $endgroup$
















                          0












                          0








                          0





                          $begingroup$

                          This is a very good question because this is exactly what early algebraic topologists cared about! The general case is no; there are no conditions on homology that are sufficient to say a space is contractible. The double comb space (https://topospaces.subwiki.org/wiki/Double_comb_space) is a space whose homology (and homotopy) groups are all trivial with coefficients in any group. It also is not contractible meaning it is not homotopy equivalent to a point.



                          But when you have a great question, a counterexample should not dissuade you. Can we put restrictions on a space so that trivial homology (with coefficients in integers) implies it is contractible? The answer is yes. If we restrict to CW complexes, you can prove that any map that induces an isomorphism on all homotopy groups must be a homotopy equivalence. This is called Whitehead's theorem. One of its corollaries is that between simply connected CW complexes, any map that induces isomorphisms on homology groups is a homotopy equivalence. This means that a simply connected CW complex with trivial homology is contractible since the map to a point induces isomorphisms on homology.






                          share|cite|improve this answer









                          $endgroup$



                          This is a very good question because this is exactly what early algebraic topologists cared about! The general case is no; there are no conditions on homology that are sufficient to say a space is contractible. The double comb space (https://topospaces.subwiki.org/wiki/Double_comb_space) is a space whose homology (and homotopy) groups are all trivial with coefficients in any group. It also is not contractible meaning it is not homotopy equivalent to a point.



                          But when you have a great question, a counterexample should not dissuade you. Can we put restrictions on a space so that trivial homology (with coefficients in integers) implies it is contractible? The answer is yes. If we restrict to CW complexes, you can prove that any map that induces an isomorphism on all homotopy groups must be a homotopy equivalence. This is called Whitehead's theorem. One of its corollaries is that between simply connected CW complexes, any map that induces isomorphisms on homology groups is a homotopy equivalence. This means that a simply connected CW complex with trivial homology is contractible since the map to a point induces isomorphisms on homology.







                          share|cite|improve this answer












                          share|cite|improve this answer



                          share|cite|improve this answer










                          answered 3 hours ago









                          Connor MalinConnor Malin

                          584111




                          584111






























                              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%2f3168982%2fnecessary-condition-on-homology-group-for-a-set-to-be-contractible%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

                              A CLEAN and SIMPLE way to add appendices to Table of Contents and bookmarks

                              Calculate evaluation metrics using cross_val_predict sklearn

                              Insert data from modal to MySQL (multiple modal on website)