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nLab > Latest Changes: group cohomology

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  1.  
    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeMar 3rd 2010
    • (edited Oct 1st 2012)

    added to group cohomology

    • in the section structured group cohomology some remarks about how to correctly define Lie group cohomology and topological group cohomology etc. and how not to

    • in the section Lie group cohiomology a derivation of how from the right oo-categorical definition one finds after some unwinding the correct definition as given in the article by Brylinski cited there.

    it's late here and I am now in a bit of a hurry to call it quits, so the proof I give there may need a bit polishing. I'll take care of that later...

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeMar 9th 2010
    • (edited Mar 9th 2010)

    two more propositions at group cohomology Lie and topological group cohomology and two more references.

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeSep 15th 2011

    In reply to this MO question I have added to group cohomology a bunch of references for the group cohomology of various topological groups.

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeOct 1st 2012
    • (edited Oct 1st 2012)

    Promted by discussion in another thread I noticed that group cohomology still only had an Idea-section, no Definition-section.

    It still really hasn’t, but I started adding something.

    So I jotted down the definitions

    1. In homotopy type theory

    2. In homological algebra

    It is fun to notice that the syntax in homotopy type theory

    ∏ x:BG(*→A) \prod_{x \colon \mathbf{B}G} (* \to A)

    is so very close to that in homological algebra

    Ext ℤG(ℤ,A) Ext_{\mathbb{Z}G}(\mathbb{Z}, A )

    (but of course vastly more general).

    I quit now. Not because I mean to suggest that what I added is done in any way, but because I have to quit. I’ll try to expand on this and turn it into something actually readable tomorrow.

    • CommentRowNumber5.
    • CommentAuthorTodd_Trimble
    • CommentTimeOct 1st 2012

    Note to self that we might add some words about Galois cohomology.

    • CommentRowNumber6.
    • CommentAuthorTim_Porter
    • CommentTimeOct 2nd 2012
    • (edited Oct 2nd 2012)

    Todd: Perhaps Galois cohomology should be a separate entry with a link from group cohomology. (I looked at the Wikipedia entry on Galois cohomology…. ! )

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeOct 3rd 2012

    I have been further working on the entry group cohomology.

    I am still not really happy with it, but I think now it is at least taking shape.

    I have entirely rewritten the Idea-section (same idea, but nevertheless rewritten) and tried to streamline various things following it.

    The main thing missing now, to my mind, is more details that unwind the abstract definitions, beyond the case of dgree-2 that is already spelled out in some detail. Parts of this I am going to import from what is currently at projective resolution, parts of it still need to be written.

    • CommentRowNumber8.
    • CommentAuthorUrs
    • CommentTimeOct 4th 2012

    tried to clean up what is currently the section Simplicial constructions by giving it a more systematic organization.

    • CommentRowNumber9.
    • CommentAuthorUrs
    • CommentTimeOct 9th 2012
    • (edited Oct 9th 2012)

    I have expanded the section Degree-2 group cohomology (which spells out the explicit component formulas) and added a discussion of how every gorup 2-cocycle is cohomologous to a normalized one (scroll down a bit to see this).

    • CommentRowNumber10.
    • CommentAuthorUrs
    • CommentTimeOct 29th 2012

    added to the Definition-section at group cohomology remarks on the relation to the homotopy-version/derived functor of the invariants functor

    (very brief remarks though. This whole section needs to be exapanded, eventually)

    • CommentRowNumber11.
    • CommentAuthorUrs
    • CommentTimeSep 26th 2014

    added pointer to Cadek 99 where the cohomology of BO(n)B O(n) with twisted integer coefficients is given. (Happened to need that.)

    • CommentRowNumber12.
    • CommentAuthorUrs
    • CommentTimeApr 16th 2015
    • (edited Apr 16th 2015)

    I have adjusted a little bit the formatting of the maps of tetrahedra at group cohomology – in degree 2 (if anyone recognizes any tetrahedra there…). Of course it still looks pitiful, but a shade of pitifulness less than it did before.

  13. added a relationship between group cohomology and comonoid

    Anonymous

    diff, v65, current

  14. added a relationship between group cohomology and comonoid

    Anonymous

    diff, v66, current

    • CommentRowNumber15.
    • CommentAuthorDmitri Pavlov
    • CommentTimeApr 5th 2021

    Added:

    Original references

    • S. Eilenberg, S. MacLane, Relations between Homology and Homotopy Groups. Proceedings of the National Academy of Sciences 29:5 (1943), 155–158. doi.

    • Samuel Eilenberg, Saunders MacLane, Relations Between Homology and Homotopy Groups of Spaces. The Annals of Mathematics 46:3 (1945), 480. doi.

    • Samuel Eilenberg, Saunders MacLane, Relations Between Homology and Homotopy Groups of Spaces. II. The Annals of Mathematics 51:3 (1950), 514. doi.

    • Dmitri Faddeev, On factor-systems in Abelian groups with operators. (Russian), Doklady Akad. Nauk SSSR (N. S.) 58, (1947). 361–364.

    diff, v67, current

  16. fixed typo in computation of group cohomology via projective resolution

    Anonymous

    diff, v75, current

    • CommentRowNumber17.
    • CommentAuthorUrs
    • CommentTimeApr 17th 2023

    added pointer to:

    diff, v76, current

    • CommentRowNumber18.
    • CommentAuthorUrs
    • CommentTimeJun 22nd 2023

    added pointer to:

    diff, v77, current

    • CommentRowNumber19.
    • CommentAuthorperezl.alonso
    • CommentTimeNov 24th 2023

    cohomology of ℤ 2 n\mathbb{Z}_2^n with coefficients in ℤ 2\mathbb{Z}_2

    diff, v78, current

1 to 19 of 19

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