Necessary condition on homology group for a set to be contractible
$begingroup$
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?
algebraic-topology simplicial-complex
$endgroup$
add a comment |
$begingroup$
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?
algebraic-topology simplicial-complex
$endgroup$
add a comment |
$begingroup$
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?
algebraic-topology simplicial-complex
$endgroup$
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
algebraic-topology simplicial-complex
asked Mar 31 at 2:07
Sanae KochiyaSanae Kochiya
676
676
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
add a comment |
$begingroup$
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).
$endgroup$
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
add a comment |
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
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
add a comment |
$begingroup$
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.
$endgroup$
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.
answered Mar 31 at 3:13
WilliamWilliam
3,1521227
3,1521227
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
add a comment |
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
$begingroup$
Thank you for your detailed explanation. My unfamiliarity with CW complex and corollaries with Whitehead' s theorem (I have not heard the version you mentioned) is the reason why I had this problem.
$endgroup$
– Sanae Kochiya
Mar 31 at 15:19
add a comment |
$begingroup$
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).
$endgroup$
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
add a comment |
$begingroup$
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).
$endgroup$
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
add a comment |
$begingroup$
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).
$endgroup$
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).
answered Mar 31 at 2:45
hunterhunter
15.6k32642
15.6k32642
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
add a comment |
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
Thank you for your response. Do you mind direct me to the proof of your statement?
$endgroup$
– Sanae Kochiya
Mar 31 at 2:54
$begingroup$
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
Mar 31 at 3:40
$begingroup$
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
Mar 31 at 3:40
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
add a comment |
$begingroup$
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.
$endgroup$
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
add a comment |
$begingroup$
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.
$endgroup$
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.
answered Mar 31 at 3:18
Connor MalinConnor Malin
584111
584111
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
add a comment |
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
$begingroup$
Thank you for your comments. Unfortunately, I am not very familiar with CW complex and I could have figured this out by myself ....
$endgroup$
– Sanae Kochiya
Mar 31 at 15:16
add a comment |
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.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
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
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
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