How does the image of the Hurewicz map $pi_n(X,x) to H_n(X)$ depend upon the choice of the base point?












3












$begingroup$


Let $X$ be a path connected topological space. I understand that the homotopy groups $pi_n(X,x_0)$ and $pi_n(X,x_1)$ are isomorphic to each other. However I do not understand whether the image of the Hurewicz map $pi_n(X,x) to H_n(X)$ is dependent or independent of the choice of basepoint. Is there any easy way to understand this ? Apologies if I am asking something sily.
I would greatly appreciate any references. Thanks.










share|cite|improve this question









$endgroup$












  • $begingroup$
    Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:00












  • $begingroup$
    If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:09










  • $begingroup$
    @Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
    $endgroup$
    – user90041
    Dec 19 '18 at 21:30










  • $begingroup$
    It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 21:57












  • $begingroup$
    If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 22:03


















3












$begingroup$


Let $X$ be a path connected topological space. I understand that the homotopy groups $pi_n(X,x_0)$ and $pi_n(X,x_1)$ are isomorphic to each other. However I do not understand whether the image of the Hurewicz map $pi_n(X,x) to H_n(X)$ is dependent or independent of the choice of basepoint. Is there any easy way to understand this ? Apologies if I am asking something sily.
I would greatly appreciate any references. Thanks.










share|cite|improve this question









$endgroup$












  • $begingroup$
    Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:00












  • $begingroup$
    If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:09










  • $begingroup$
    @Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
    $endgroup$
    – user90041
    Dec 19 '18 at 21:30










  • $begingroup$
    It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 21:57












  • $begingroup$
    If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 22:03
















3












3








3


1



$begingroup$


Let $X$ be a path connected topological space. I understand that the homotopy groups $pi_n(X,x_0)$ and $pi_n(X,x_1)$ are isomorphic to each other. However I do not understand whether the image of the Hurewicz map $pi_n(X,x) to H_n(X)$ is dependent or independent of the choice of basepoint. Is there any easy way to understand this ? Apologies if I am asking something sily.
I would greatly appreciate any references. Thanks.










share|cite|improve this question









$endgroup$




Let $X$ be a path connected topological space. I understand that the homotopy groups $pi_n(X,x_0)$ and $pi_n(X,x_1)$ are isomorphic to each other. However I do not understand whether the image of the Hurewicz map $pi_n(X,x) to H_n(X)$ is dependent or independent of the choice of basepoint. Is there any easy way to understand this ? Apologies if I am asking something sily.
I would greatly appreciate any references. Thanks.







algebraic-topology homotopy-theory higher-homotopy-groups






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 19 '18 at 18:30









user90041user90041

1,8141335




1,8141335












  • $begingroup$
    Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:00












  • $begingroup$
    If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:09










  • $begingroup$
    @Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
    $endgroup$
    – user90041
    Dec 19 '18 at 21:30










  • $begingroup$
    It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 21:57












  • $begingroup$
    If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 22:03




















  • $begingroup$
    Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:00












  • $begingroup$
    If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 20:09










  • $begingroup$
    @Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
    $endgroup$
    – user90041
    Dec 19 '18 at 21:30










  • $begingroup$
    It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 21:57












  • $begingroup$
    If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
    $endgroup$
    – Tyrone
    Dec 19 '18 at 22:03


















$begingroup$
Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
$endgroup$
– Tyrone
Dec 19 '18 at 20:00






$begingroup$
Notet that if $X=bigsqcup X_i$ is a disjoint union of connected components and $X_0$ is the component of the basepoint, then $pi_n(X)=pi_n(X_0)$ for $ngeq 1$. On the other hand $H_n(X)congbigoplus H_n(X_i)$. Therefore if $X$ is not connected then there is a lot of dependence on the choice of basepoint, since the Hurewicz homomorphism can only have image in $H_n(X_0)$.
$endgroup$
– Tyrone
Dec 19 '18 at 20:00














$begingroup$
If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
$endgroup$
– Tyrone
Dec 19 '18 at 20:09




$begingroup$
If $X$ is path connected then there is no dependence on the choice of basepoint, since choosing a path between any two points gives you an isomorphism of the homotopy groups based at those two points.
$endgroup$
– Tyrone
Dec 19 '18 at 20:09












$begingroup$
@Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
$endgroup$
– user90041
Dec 19 '18 at 21:30




$begingroup$
@Tyrone Thanks for your comment. I agree with your first comment about the case when $X$ has several components. I further understand that there is an isomorphism between homotopy groups corresponding to the two base points. In general the isomorphism depends on the homotopy class of the path connecting the two basepoints. It is not clear whether any such isomorphism is compatible (i.e. results in a commutative diagram) with the two Hurewicz maps. Am I missing something here ?
$endgroup$
– user90041
Dec 19 '18 at 21:30












$begingroup$
It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
$endgroup$
– Tyrone
Dec 19 '18 at 21:57






$begingroup$
It's not exactly clear, which is why I left a comment and not an answer. Here are some thoughts. The isomorphism uses the fact that the basepoint inclusion $asthookrightarrow S^n$ is a closed cofibration ($S^n$ is CW). A path $l:Irightarrow X$ from $x_0$ to $x_1$ can be thought of as a homotopy $asttimes Irightarrow X$. Then the HEP gives, for any $alpha:S^nrightarrow X$ based at $x_0$, an unbased homotopy $A:S^ntimes Irightarrow X$, starting at $alpha$ and ending at a map $alpha_1:S^nrightarrow X$ based at $x_1$. This is the aforementioned homomorphism.
$endgroup$
– Tyrone
Dec 19 '18 at 21:57














$begingroup$
If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
$endgroup$
– Tyrone
Dec 19 '18 at 22:03






$begingroup$
If $X$ is path connected and $pi_1X=0$ (for some basepoint) then unbased homotopy is equivalent to based homotopy (for any choice of basepoint). If $pi_1Xneq 0$ then the previous procedure defines an action of $pi_1X$ on $pi_nX$. As long as $X$ is connected there are still (non-canonical) isomorphisms between the $pi_1$s with various basepoints. These *should* be related by the $pi_1$-action, and the image should be the same. If no one beats me to it I'll try to write an answer up tomorrow when I have time.
$endgroup$
– Tyrone
Dec 19 '18 at 22:03












2 Answers
2






active

oldest

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3












$begingroup$

Note that we don't just have some arbitrary isomorphism $pi_n(X,x_1)to pi_n(X,x_0)$; we have an explicit description of what the map is. Namely, we can get such an isomorphism by picking a path $gamma$ from $x_0$ to $x_1$ and then inserting copies of $gamma$ radially starting at the basepoint $s_0$ of $S^n$ to turn a map $f:(S^n,s_0)to (X,x_1)$ into a map $f^gamma:(S^n,s_0)to (X,x_0)$. Now the key observation is that this map $f^gamma$ is actually homotopic to $f$ as a map $S^nto X$ (i.e., ignoring the basepoints). The homotopy is messy to write down explicitly but easily to visualize: you just gradually shrink the radial extensions, using only the portion between $gamma(t)$ and $x_1=gamma(1)$ for the $t$th step of the homotopy (so the $t$th step maps $s_0$ to $gamma(t)$). In terms of the picture at the top of page 341 of Hatcher's Algebraic Topology, the intermediate stages of the homotopy are given by restricting to squares which are intermediate between the inner $f$ square and the full outer square.



In particular, this means $f$ and $f^gamma$ induce the same map on $H_n$. Since the image of $f$ under the Hurewicz map is just the image of the fundamental class in $H_n(S^n)$ under $f$, this means that $f$ and $f^gamma$ have the same Hurewicz image. It follows that the Hurewicz images of $pi_n(X,x_1)$ and $pi_n(X,x_0)$ are the same.






share|cite|improve this answer











$endgroup$





















    1












    $begingroup$

    By naturality, it suffices to examine the universal example $X = S^n$. In this case, the Hurewicz homomorphism $pi_n(S^n, s) to H_n(S^n)$ is an isomorphism for any base point $s$, so in particular the image does not depend on the base point.





    EDIT: Here is the naturality argument I had in mind. Let $f in pi_n(X, x)$ correspond to a map $f: (S^n, s) to (X, x)$. Then we have
    $$require{AMScd}
    begin{CD}
    pi_n(S^n, s) @>cong>> H_n(S^n) \
    @V{f_*}VV @VV{f_*}V \
    pi_n(X, x) @>>h> H_n(X)
    end{CD}
    $$



    We have $h(f) = f_*(1)$, for $f_*$ the induced map on homology which doesn't care about basepoints. This reduces the question to the issue of whether the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps in homology. Homology doesn't care about basepoints, and forgetting base points, we get the same classes of maps up to homotopy provided $x_1$ and $x_2$ are in the same path component of $X$, as $S^n$ is connected for $n > 0$.






    share|cite|improve this answer











    $endgroup$













    • $begingroup$
      Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
      $endgroup$
      – user90041
      Dec 19 '18 at 19:18












    • $begingroup$
      ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
      $endgroup$
      – Tyrone
      Dec 19 '18 at 19:51






    • 1




      $begingroup$
      I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
      $endgroup$
      – Eric Wofsey
      Dec 19 '18 at 22:19












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    2 Answers
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    2 Answers
    2






    active

    oldest

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    active

    oldest

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    active

    oldest

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    3












    $begingroup$

    Note that we don't just have some arbitrary isomorphism $pi_n(X,x_1)to pi_n(X,x_0)$; we have an explicit description of what the map is. Namely, we can get such an isomorphism by picking a path $gamma$ from $x_0$ to $x_1$ and then inserting copies of $gamma$ radially starting at the basepoint $s_0$ of $S^n$ to turn a map $f:(S^n,s_0)to (X,x_1)$ into a map $f^gamma:(S^n,s_0)to (X,x_0)$. Now the key observation is that this map $f^gamma$ is actually homotopic to $f$ as a map $S^nto X$ (i.e., ignoring the basepoints). The homotopy is messy to write down explicitly but easily to visualize: you just gradually shrink the radial extensions, using only the portion between $gamma(t)$ and $x_1=gamma(1)$ for the $t$th step of the homotopy (so the $t$th step maps $s_0$ to $gamma(t)$). In terms of the picture at the top of page 341 of Hatcher's Algebraic Topology, the intermediate stages of the homotopy are given by restricting to squares which are intermediate between the inner $f$ square and the full outer square.



    In particular, this means $f$ and $f^gamma$ induce the same map on $H_n$. Since the image of $f$ under the Hurewicz map is just the image of the fundamental class in $H_n(S^n)$ under $f$, this means that $f$ and $f^gamma$ have the same Hurewicz image. It follows that the Hurewicz images of $pi_n(X,x_1)$ and $pi_n(X,x_0)$ are the same.






    share|cite|improve this answer











    $endgroup$


















      3












      $begingroup$

      Note that we don't just have some arbitrary isomorphism $pi_n(X,x_1)to pi_n(X,x_0)$; we have an explicit description of what the map is. Namely, we can get such an isomorphism by picking a path $gamma$ from $x_0$ to $x_1$ and then inserting copies of $gamma$ radially starting at the basepoint $s_0$ of $S^n$ to turn a map $f:(S^n,s_0)to (X,x_1)$ into a map $f^gamma:(S^n,s_0)to (X,x_0)$. Now the key observation is that this map $f^gamma$ is actually homotopic to $f$ as a map $S^nto X$ (i.e., ignoring the basepoints). The homotopy is messy to write down explicitly but easily to visualize: you just gradually shrink the radial extensions, using only the portion between $gamma(t)$ and $x_1=gamma(1)$ for the $t$th step of the homotopy (so the $t$th step maps $s_0$ to $gamma(t)$). In terms of the picture at the top of page 341 of Hatcher's Algebraic Topology, the intermediate stages of the homotopy are given by restricting to squares which are intermediate between the inner $f$ square and the full outer square.



      In particular, this means $f$ and $f^gamma$ induce the same map on $H_n$. Since the image of $f$ under the Hurewicz map is just the image of the fundamental class in $H_n(S^n)$ under $f$, this means that $f$ and $f^gamma$ have the same Hurewicz image. It follows that the Hurewicz images of $pi_n(X,x_1)$ and $pi_n(X,x_0)$ are the same.






      share|cite|improve this answer











      $endgroup$
















        3












        3








        3





        $begingroup$

        Note that we don't just have some arbitrary isomorphism $pi_n(X,x_1)to pi_n(X,x_0)$; we have an explicit description of what the map is. Namely, we can get such an isomorphism by picking a path $gamma$ from $x_0$ to $x_1$ and then inserting copies of $gamma$ radially starting at the basepoint $s_0$ of $S^n$ to turn a map $f:(S^n,s_0)to (X,x_1)$ into a map $f^gamma:(S^n,s_0)to (X,x_0)$. Now the key observation is that this map $f^gamma$ is actually homotopic to $f$ as a map $S^nto X$ (i.e., ignoring the basepoints). The homotopy is messy to write down explicitly but easily to visualize: you just gradually shrink the radial extensions, using only the portion between $gamma(t)$ and $x_1=gamma(1)$ for the $t$th step of the homotopy (so the $t$th step maps $s_0$ to $gamma(t)$). In terms of the picture at the top of page 341 of Hatcher's Algebraic Topology, the intermediate stages of the homotopy are given by restricting to squares which are intermediate between the inner $f$ square and the full outer square.



        In particular, this means $f$ and $f^gamma$ induce the same map on $H_n$. Since the image of $f$ under the Hurewicz map is just the image of the fundamental class in $H_n(S^n)$ under $f$, this means that $f$ and $f^gamma$ have the same Hurewicz image. It follows that the Hurewicz images of $pi_n(X,x_1)$ and $pi_n(X,x_0)$ are the same.






        share|cite|improve this answer











        $endgroup$



        Note that we don't just have some arbitrary isomorphism $pi_n(X,x_1)to pi_n(X,x_0)$; we have an explicit description of what the map is. Namely, we can get such an isomorphism by picking a path $gamma$ from $x_0$ to $x_1$ and then inserting copies of $gamma$ radially starting at the basepoint $s_0$ of $S^n$ to turn a map $f:(S^n,s_0)to (X,x_1)$ into a map $f^gamma:(S^n,s_0)to (X,x_0)$. Now the key observation is that this map $f^gamma$ is actually homotopic to $f$ as a map $S^nto X$ (i.e., ignoring the basepoints). The homotopy is messy to write down explicitly but easily to visualize: you just gradually shrink the radial extensions, using only the portion between $gamma(t)$ and $x_1=gamma(1)$ for the $t$th step of the homotopy (so the $t$th step maps $s_0$ to $gamma(t)$). In terms of the picture at the top of page 341 of Hatcher's Algebraic Topology, the intermediate stages of the homotopy are given by restricting to squares which are intermediate between the inner $f$ square and the full outer square.



        In particular, this means $f$ and $f^gamma$ induce the same map on $H_n$. Since the image of $f$ under the Hurewicz map is just the image of the fundamental class in $H_n(S^n)$ under $f$, this means that $f$ and $f^gamma$ have the same Hurewicz image. It follows that the Hurewicz images of $pi_n(X,x_1)$ and $pi_n(X,x_0)$ are the same.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 19 '18 at 22:16

























        answered Dec 19 '18 at 22:11









        Eric WofseyEric Wofsey

        192k14217351




        192k14217351























            1












            $begingroup$

            By naturality, it suffices to examine the universal example $X = S^n$. In this case, the Hurewicz homomorphism $pi_n(S^n, s) to H_n(S^n)$ is an isomorphism for any base point $s$, so in particular the image does not depend on the base point.





            EDIT: Here is the naturality argument I had in mind. Let $f in pi_n(X, x)$ correspond to a map $f: (S^n, s) to (X, x)$. Then we have
            $$require{AMScd}
            begin{CD}
            pi_n(S^n, s) @>cong>> H_n(S^n) \
            @V{f_*}VV @VV{f_*}V \
            pi_n(X, x) @>>h> H_n(X)
            end{CD}
            $$



            We have $h(f) = f_*(1)$, for $f_*$ the induced map on homology which doesn't care about basepoints. This reduces the question to the issue of whether the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps in homology. Homology doesn't care about basepoints, and forgetting base points, we get the same classes of maps up to homotopy provided $x_1$ and $x_2$ are in the same path component of $X$, as $S^n$ is connected for $n > 0$.






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
              $endgroup$
              – user90041
              Dec 19 '18 at 19:18












            • $begingroup$
              ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
              $endgroup$
              – Tyrone
              Dec 19 '18 at 19:51






            • 1




              $begingroup$
              I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
              $endgroup$
              – Eric Wofsey
              Dec 19 '18 at 22:19
















            1












            $begingroup$

            By naturality, it suffices to examine the universal example $X = S^n$. In this case, the Hurewicz homomorphism $pi_n(S^n, s) to H_n(S^n)$ is an isomorphism for any base point $s$, so in particular the image does not depend on the base point.





            EDIT: Here is the naturality argument I had in mind. Let $f in pi_n(X, x)$ correspond to a map $f: (S^n, s) to (X, x)$. Then we have
            $$require{AMScd}
            begin{CD}
            pi_n(S^n, s) @>cong>> H_n(S^n) \
            @V{f_*}VV @VV{f_*}V \
            pi_n(X, x) @>>h> H_n(X)
            end{CD}
            $$



            We have $h(f) = f_*(1)$, for $f_*$ the induced map on homology which doesn't care about basepoints. This reduces the question to the issue of whether the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps in homology. Homology doesn't care about basepoints, and forgetting base points, we get the same classes of maps up to homotopy provided $x_1$ and $x_2$ are in the same path component of $X$, as $S^n$ is connected for $n > 0$.






            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
              $endgroup$
              – user90041
              Dec 19 '18 at 19:18












            • $begingroup$
              ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
              $endgroup$
              – Tyrone
              Dec 19 '18 at 19:51






            • 1




              $begingroup$
              I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
              $endgroup$
              – Eric Wofsey
              Dec 19 '18 at 22:19














            1












            1








            1





            $begingroup$

            By naturality, it suffices to examine the universal example $X = S^n$. In this case, the Hurewicz homomorphism $pi_n(S^n, s) to H_n(S^n)$ is an isomorphism for any base point $s$, so in particular the image does not depend on the base point.





            EDIT: Here is the naturality argument I had in mind. Let $f in pi_n(X, x)$ correspond to a map $f: (S^n, s) to (X, x)$. Then we have
            $$require{AMScd}
            begin{CD}
            pi_n(S^n, s) @>cong>> H_n(S^n) \
            @V{f_*}VV @VV{f_*}V \
            pi_n(X, x) @>>h> H_n(X)
            end{CD}
            $$



            We have $h(f) = f_*(1)$, for $f_*$ the induced map on homology which doesn't care about basepoints. This reduces the question to the issue of whether the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps in homology. Homology doesn't care about basepoints, and forgetting base points, we get the same classes of maps up to homotopy provided $x_1$ and $x_2$ are in the same path component of $X$, as $S^n$ is connected for $n > 0$.






            share|cite|improve this answer











            $endgroup$



            By naturality, it suffices to examine the universal example $X = S^n$. In this case, the Hurewicz homomorphism $pi_n(S^n, s) to H_n(S^n)$ is an isomorphism for any base point $s$, so in particular the image does not depend on the base point.





            EDIT: Here is the naturality argument I had in mind. Let $f in pi_n(X, x)$ correspond to a map $f: (S^n, s) to (X, x)$. Then we have
            $$require{AMScd}
            begin{CD}
            pi_n(S^n, s) @>cong>> H_n(S^n) \
            @V{f_*}VV @VV{f_*}V \
            pi_n(X, x) @>>h> H_n(X)
            end{CD}
            $$



            We have $h(f) = f_*(1)$, for $f_*$ the induced map on homology which doesn't care about basepoints. This reduces the question to the issue of whether the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps in homology. Homology doesn't care about basepoints, and forgetting base points, we get the same classes of maps up to homotopy provided $x_1$ and $x_2$ are in the same path component of $X$, as $S^n$ is connected for $n > 0$.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Dec 19 '18 at 20:41

























            answered Dec 19 '18 at 18:59









            JHFJHF

            4,9611026




            4,9611026












            • $begingroup$
              Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
              $endgroup$
              – user90041
              Dec 19 '18 at 19:18












            • $begingroup$
              ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
              $endgroup$
              – Tyrone
              Dec 19 '18 at 19:51






            • 1




              $begingroup$
              I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
              $endgroup$
              – Eric Wofsey
              Dec 19 '18 at 22:19


















            • $begingroup$
              Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
              $endgroup$
              – user90041
              Dec 19 '18 at 19:18












            • $begingroup$
              ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
              $endgroup$
              – Tyrone
              Dec 19 '18 at 19:51






            • 1




              $begingroup$
              I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
              $endgroup$
              – Eric Wofsey
              Dec 19 '18 at 22:19
















            $begingroup$
            Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
            $endgroup$
            – user90041
            Dec 19 '18 at 19:18






            $begingroup$
            Thanks. But I am sorry that I do not understand. In the category of pointed spaces, our objects are $(S^n,s)$, $(X,x_1)$ and $(X,x_2)$. I let $f_i : (S^n,s) to (X,x_i)$ be two maps, and draw the naturality diagram. But after that, it is not clear to me how diagram chasing implies that the statement, since neither of the induced maps on the homotpy groups or homology groups are surjective.
            $endgroup$
            – user90041
            Dec 19 '18 at 19:18














            $begingroup$
            ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
            $endgroup$
            – Tyrone
            Dec 19 '18 at 19:51




            $begingroup$
            ..but the image clearly does depend on the choice of basepoint if $X$ is not connected...
            $endgroup$
            – Tyrone
            Dec 19 '18 at 19:51




            1




            1




            $begingroup$
            I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
            $endgroup$
            – Eric Wofsey
            Dec 19 '18 at 22:19




            $begingroup$
            I don't see how it's obvious that the classes the classes ${f: (S^n, s) to (X, x_1)}$ and ${f: (S^n, s) to (X, x_2)}$ induce the same collection of maps on homology. After all, these are different maps $S^nto X$. You have to actually show how to turn a map in one class into a map in the other class which induces the same map on homology.
            $endgroup$
            – Eric Wofsey
            Dec 19 '18 at 22:19


















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