Proof regarding n-connectedness












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If one has to prove that $R^n - {0}$ is not $(n-1)$-connected, is it necessary to prove formally that there exists a non contractible $(n-1)$-sphere or can that simply be stated. If one must formally prove this, how may that be done? Can one, for example, consider the intersection of the $(n-1)$-sphere and $R^n$ with an arbitrary 2D plane and then say (for the sake of contradiction by the definition of 1-connectedness) that the given sphere is contractible if and only if the circle that corresponds to the intersection of that sphere is contractible in the subspace topology defined by the intersection of the plane and $R^n$?






general-topology proof-verification geometric-topology







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  • I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
    – Babelfish
    Jan 4 at 8:22


















0














If one has to prove that $R^n - {0}$ is not $(n-1)$-connected, is it necessary to prove formally that there exists a non contractible $(n-1)$-sphere or can that simply be stated. If one must formally prove this, how may that be done? Can one, for example, consider the intersection of the $(n-1)$-sphere and $R^n$ with an arbitrary 2D plane and then say (for the sake of contradiction by the definition of 1-connectedness) that the given sphere is contractible if and only if the circle that corresponds to the intersection of that sphere is contractible in the subspace topology defined by the intersection of the plane and $R^n$?






general-topology proof-verification geometric-topology







share|cite|improve this question
























  • I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
    – Babelfish
    Jan 4 at 8:22
















0












0








0


1





If one has to prove that $R^n - {0}$ is not $(n-1)$-connected, is it necessary to prove formally that there exists a non contractible $(n-1)$-sphere or can that simply be stated. If one must formally prove this, how may that be done? Can one, for example, consider the intersection of the $(n-1)$-sphere and $R^n$ with an arbitrary 2D plane and then say (for the sake of contradiction by the definition of 1-connectedness) that the given sphere is contractible if and only if the circle that corresponds to the intersection of that sphere is contractible in the subspace topology defined by the intersection of the plane and $R^n$?






general-topology proof-verification geometric-topology







share|cite|improve this question















If one has to prove that $R^n - {0}$ is not $(n-1)$-connected, is it necessary to prove formally that there exists a non contractible $(n-1)$-sphere or can that simply be stated. If one must formally prove this, how may that be done? Can one, for example, consider the intersection of the $(n-1)$-sphere and $R^n$ with an arbitrary 2D plane and then say (for the sake of contradiction by the definition of 1-connectedness) that the given sphere is contractible if and only if the circle that corresponds to the intersection of that sphere is contractible in the subspace topology defined by the intersection of the plane and $R^n$?






general-topology proof-verification geometric-topology




general-topology proof-verification geometric-topology






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edited yesterday









Paul Frost

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asked Jan 4 at 8:11









Aryaman GuptaAryaman Gupta

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  • I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
    – Babelfish
    Jan 4 at 8:22




















  • I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
    – Babelfish
    Jan 4 at 8:22


















I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
– Babelfish
Jan 4 at 8:22






I think you have an index shift in the first line. $R^n setminus{0}$ is $n-1$-connected. $Rsetminus {0}$ is not connected, $R^2setminus {0}$ is not simply connected (i.e. not $1$-connected).
– Babelfish
Jan 4 at 8:22












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This is not immanent in the definitions, so there is need for a proof.



Your approach, however, is not sufficient. In general, there could be a contraction of the $n-1$-sphere outside the plane, without the existence of a contraction inside the plane. I think your approach would say something about relative homotopy groups.



The standard approach (at least I don't know an easier way) to show that $R^nsetminus {0}$ is not $n-1$-connected goes by first showing that $R^n setminus {0} simeq S^{n-1}$ (homotopy equivalence). Therefore, all homotopy groups will be isomorphic.



So it remains to show that $S^{n-1}$ is not $n-1$-connected. It is well known that $S^{n-1}$ is $n-2$-connected, so by the Hurewicz theorem, $pi_{n-1}(S^{n-1}) cong H_{n-1}(S^{n-1})$. Homology is not so hard to calculate, in particular $H_{n-1}(S^{n-1})cong mathbb{Z}neq 0$, so $S^{n-1}$ is not ${n-1}$-connected.
See also Wikipedia for this.






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  • Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
    – Babelfish
    23 hours ago













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This is not immanent in the definitions, so there is need for a proof.



Your approach, however, is not sufficient. In general, there could be a contraction of the $n-1$-sphere outside the plane, without the existence of a contraction inside the plane. I think your approach would say something about relative homotopy groups.



The standard approach (at least I don't know an easier way) to show that $R^nsetminus {0}$ is not $n-1$-connected goes by first showing that $R^n setminus {0} simeq S^{n-1}$ (homotopy equivalence). Therefore, all homotopy groups will be isomorphic.



So it remains to show that $S^{n-1}$ is not $n-1$-connected. It is well known that $S^{n-1}$ is $n-2$-connected, so by the Hurewicz theorem, $pi_{n-1}(S^{n-1}) cong H_{n-1}(S^{n-1})$. Homology is not so hard to calculate, in particular $H_{n-1}(S^{n-1})cong mathbb{Z}neq 0$, so $S^{n-1}$ is not ${n-1}$-connected.
See also Wikipedia for this.






share|cite|improve this answer





















  • Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
    – Babelfish
    23 hours ago


















0














This is not immanent in the definitions, so there is need for a proof.



Your approach, however, is not sufficient. In general, there could be a contraction of the $n-1$-sphere outside the plane, without the existence of a contraction inside the plane. I think your approach would say something about relative homotopy groups.



The standard approach (at least I don't know an easier way) to show that $R^nsetminus {0}$ is not $n-1$-connected goes by first showing that $R^n setminus {0} simeq S^{n-1}$ (homotopy equivalence). Therefore, all homotopy groups will be isomorphic.



So it remains to show that $S^{n-1}$ is not $n-1$-connected. It is well known that $S^{n-1}$ is $n-2$-connected, so by the Hurewicz theorem, $pi_{n-1}(S^{n-1}) cong H_{n-1}(S^{n-1})$. Homology is not so hard to calculate, in particular $H_{n-1}(S^{n-1})cong mathbb{Z}neq 0$, so $S^{n-1}$ is not ${n-1}$-connected.
See also Wikipedia for this.






share|cite|improve this answer





















  • Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
    – Babelfish
    23 hours ago
















0












0








0






This is not immanent in the definitions, so there is need for a proof.



Your approach, however, is not sufficient. In general, there could be a contraction of the $n-1$-sphere outside the plane, without the existence of a contraction inside the plane. I think your approach would say something about relative homotopy groups.



The standard approach (at least I don't know an easier way) to show that $R^nsetminus {0}$ is not $n-1$-connected goes by first showing that $R^n setminus {0} simeq S^{n-1}$ (homotopy equivalence). Therefore, all homotopy groups will be isomorphic.



So it remains to show that $S^{n-1}$ is not $n-1$-connected. It is well known that $S^{n-1}$ is $n-2$-connected, so by the Hurewicz theorem, $pi_{n-1}(S^{n-1}) cong H_{n-1}(S^{n-1})$. Homology is not so hard to calculate, in particular $H_{n-1}(S^{n-1})cong mathbb{Z}neq 0$, so $S^{n-1}$ is not ${n-1}$-connected.
See also Wikipedia for this.






share|cite|improve this answer












This is not immanent in the definitions, so there is need for a proof.



Your approach, however, is not sufficient. In general, there could be a contraction of the $n-1$-sphere outside the plane, without the existence of a contraction inside the plane. I think your approach would say something about relative homotopy groups.



The standard approach (at least I don't know an easier way) to show that $R^nsetminus {0}$ is not $n-1$-connected goes by first showing that $R^n setminus {0} simeq S^{n-1}$ (homotopy equivalence). Therefore, all homotopy groups will be isomorphic.



So it remains to show that $S^{n-1}$ is not $n-1$-connected. It is well known that $S^{n-1}$ is $n-2$-connected, so by the Hurewicz theorem, $pi_{n-1}(S^{n-1}) cong H_{n-1}(S^{n-1})$. Homology is not so hard to calculate, in particular $H_{n-1}(S^{n-1})cong mathbb{Z}neq 0$, so $S^{n-1}$ is not ${n-1}$-connected.
See also Wikipedia for this.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 4 at 8:45









BabelfishBabelfish

1,126520




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  • Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
    – Babelfish
    23 hours ago




















  • Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
    – Babelfish
    23 hours ago


















Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
– Babelfish
23 hours ago






Do you need further help with this topic? Should I extend an explanation? If you are satisfied with the answer, you might consider to accept it.
– Babelfish
23 hours ago




















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