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Consider a manifold $M$. Then, we have the notion of differential forms on $M$ and complex associated to that, denoted by $$\cdots\rightarrow \Omega^{k-1}(M)\rightarrow \Omega^k(M)\rightarrow \Omega^{k+1}(M)\rightarrow \cdots$$ giving de Rham cohomology groups $H^k_{\mathrm{dR}}(M)$ for $k\in \mathbb{N}$.

Now consider a Lie group $G$. There is a notion of cohomology of this Lie group. Ignoring the group structure, we can talk about de Rham cohomology of the underlying manifold.

Question : Is there a notion of restricted complex of differential forms, that is a sub complex $\{\widetilde{\Omega^k(G)}\}$, of the complex of differential forms $\{\Omega^k(G)\}$ of $G$, whose cohomology groups gives cohomology of the Lie group?

All that (of great importance) extra structure coming in Lie group $G$ is the multiplication (and inverse) map $G\times G\rightarrow G$. So, I am expecting this restricted differential forms to show the action of $G$ on itself.

By cohomology of the Lie group, I mean the cohomology of the underlying manifold. I do not think there is any notion of cohomology of Lie group. Even Google search does not give anything.

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    $\begingroup$ When $G$ is compact a suitable subcomplex consists of those $G$-invariant forms. When $G$ is arbitrary (connected), maybe the subcomplex of $K$-invariant forms, for $K$ maximal compact subgroup of $G$, works, but I haven't checked. $\endgroup$
    – YCor
    Commented Jul 23, 2019 at 14:02
  • $\begingroup$ @YCor if it is already written somewhere, can you please suggest reference.. I will read :) $\endgroup$ Commented Jul 23, 2019 at 14:13
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    $\begingroup$ Could you clarify what notion of Lie group cohomology you're talking about? $\endgroup$ Commented Jul 23, 2019 at 17:09
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    $\begingroup$ @praphullakoushik if you mean the manifold cohomology you have the trivial answer: take the whole complex. I guess this is not what you want. As pointed above, under some hypotheses (simply connected, etc.) the Chevalley-Eilenberg complex of its Lie algebra also computes the same cohomology and it has finite dimension. $\endgroup$ Commented Jul 23, 2019 at 17:59
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    $\begingroup$ See the motivation section in en.m.wikipedia.org/wiki/Lie_algebra_cohomology $\endgroup$ Commented Jul 23, 2019 at 18:26

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For every Lie group $G$, you have in your $\Omega_G$ the subcomplex of the left-invariant differential forms. Moreover, $G$ acts on the right on this subcomplex. It has the virtue of being finite-dimensional and isomorphic to a complex defined purely in terms of the Lie algebra, thus the computation of its cohomology is reduced to linear algebra.

On the other hand, considering a maximal compact subgroup $K\subseteq G$, since $G/K$ is contractible, by Poincare's lemma, $\Omega_G$ and $\Omega_K$ have the same cohomology.

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  • $\begingroup$ Sir, thank you for your answer... can you give reference where this is mentioned.. May be a reference which says why this cohomology of subcomplex of differential forms is same as cohomology of the Lie group. $\endgroup$ Commented Jul 23, 2019 at 15:06
  • $\begingroup$ @Praphulla Bredon's book "Geometry and topology" has a good account. $\endgroup$
    – mme
    Commented Jul 23, 2019 at 15:23
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    $\begingroup$ And the cohomology of $K$ is the cohomology of $K$-invariant forms, by an averaging argument of Cartan, so you can always reduce the computation of the cohomology of $G$ to the Lie algebra cohomology of $\mathfrak{K}$. $\endgroup$
    – Ben McKay
    Commented Jul 23, 2019 at 16:19
  • $\begingroup$ @MikeMiller yes, I see that now..Thanks,. it is in section 12 of chapter 5 named Differential forms on compact Lie groups... I did not saw this before.. $\endgroup$ Commented Jul 23, 2019 at 17:15
  • $\begingroup$ @BenMcKay Thanks for the comment.. I do not know much about cohomology of Lie algebras.. If you can suggest some reference that talks about reducing cohomology of $G$ to that of cohomology of Lie algebra of $K$, I will read that.. $\endgroup$ Commented Jul 23, 2019 at 17:20

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