Understanding why the Adams Spectral Sequence works












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I am trying to learn about the Adams Spectral Sequence and my question is basically summed up in the title.



More precisely, let $X$, $Y$, and $E$ be spectra. We have a homomorphism $[X,Y] to Hom_{E^∗E}(E^∗Y,E^∗X)$, the latter giving the first page of the Adams Spectral Sequence. But why does $Ext^∗_{E^∗E}(E^∗Y,E^∗X)$ give a "better approximation" to $[X,Y]$ than $Hom_{E^∗E}(E^∗Y,E^∗X)$?



And more generally, it seems like magic that we can get homotopy from successively taking homology. Any intuition about this?










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$endgroup$








  • 3




    $begingroup$
    Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
    $endgroup$
    – John Palmieri
    Dec 3 '18 at 6:46
















1












$begingroup$


I am trying to learn about the Adams Spectral Sequence and my question is basically summed up in the title.



More precisely, let $X$, $Y$, and $E$ be spectra. We have a homomorphism $[X,Y] to Hom_{E^∗E}(E^∗Y,E^∗X)$, the latter giving the first page of the Adams Spectral Sequence. But why does $Ext^∗_{E^∗E}(E^∗Y,E^∗X)$ give a "better approximation" to $[X,Y]$ than $Hom_{E^∗E}(E^∗Y,E^∗X)$?



And more generally, it seems like magic that we can get homotopy from successively taking homology. Any intuition about this?










share|cite|improve this question









$endgroup$








  • 3




    $begingroup$
    Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
    $endgroup$
    – John Palmieri
    Dec 3 '18 at 6:46














1












1








1





$begingroup$


I am trying to learn about the Adams Spectral Sequence and my question is basically summed up in the title.



More precisely, let $X$, $Y$, and $E$ be spectra. We have a homomorphism $[X,Y] to Hom_{E^∗E}(E^∗Y,E^∗X)$, the latter giving the first page of the Adams Spectral Sequence. But why does $Ext^∗_{E^∗E}(E^∗Y,E^∗X)$ give a "better approximation" to $[X,Y]$ than $Hom_{E^∗E}(E^∗Y,E^∗X)$?



And more generally, it seems like magic that we can get homotopy from successively taking homology. Any intuition about this?










share|cite|improve this question









$endgroup$




I am trying to learn about the Adams Spectral Sequence and my question is basically summed up in the title.



More precisely, let $X$, $Y$, and $E$ be spectra. We have a homomorphism $[X,Y] to Hom_{E^∗E}(E^∗Y,E^∗X)$, the latter giving the first page of the Adams Spectral Sequence. But why does $Ext^∗_{E^∗E}(E^∗Y,E^∗X)$ give a "better approximation" to $[X,Y]$ than $Hom_{E^∗E}(E^∗Y,E^∗X)$?



And more generally, it seems like magic that we can get homotopy from successively taking homology. Any intuition about this?







algebraic-topology homology-cohomology homotopy-theory spectral-sequences






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 3 '18 at 1:13









math1234567math1234567

8661919




8661919








  • 3




    $begingroup$
    Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
    $endgroup$
    – John Palmieri
    Dec 3 '18 at 6:46














  • 3




    $begingroup$
    Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
    $endgroup$
    – John Palmieri
    Dec 3 '18 at 6:46








3




3




$begingroup$
Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
$endgroup$
– John Palmieri
Dec 3 '18 at 6:46




$begingroup$
Pretend that you understand homotopy classes of maps in one case: if the codomain is $E$, then $[X,E] = E^*X$. Then if you want to compute $[X,Y]$ for a general $Y$, you should try to approximate $Y$ by copies of $E$ somehow. This leads to the $E$-Adams tower and then to the Adams SS, which is built up from a sort of resolution of $Y$ by copies of $E$. This at least makes it plausible that Ext is relevant, rather than just Hom, or at least that homological algebra could enter into things.
$endgroup$
– John Palmieri
Dec 3 '18 at 6:46










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