# Group cohomology of orthogonal groups with integer coefficient

I would like to know the group cohomology of orthogonal groups $SO(n)$, which is the topological cohomology of the classifying space of the group: $H^*(BSO(n);\mathbb{Z}) =$ ? (for example for $n=10$)

I also like to know $H^*(BPSU(n);\mathbb{Z})$ (say for $n=3$), where $PSU(n)=SU(n)/Z_n$ and $Z_n$ is the center of $SU(n)$.

Thanks!

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Note: there exist more than one thing that one might call "group cohomology", and only one of them has the property that $H^*_{group}(G)=H^*(BG)$ (for compact $G$). –  André Henriques Feb 17 '13 at 18:21
For continuous group, the group cohomology that I am interested in is the Borel group cohomology. –  Xiao-Gang Wen Feb 18 '13 at 0:51

For a precise answer to your first question, see Theorem 1.5 of

Brown, Edgar H., Jr. The cohomology of BSOn and BOn with integer coefficients. Proc. Amer. Math. Soc. 85 (1982), no. 2, 283–288.

For your second question, note that there is an isomorphism $PSU(n)\cong PU(n)$ for each $n$, and that the cohomology $H^\ast(BPU(3);\mathbb{F}_3)$ is worked out in

Kono, Akira; Mimura, Mamoru; Shimada, Nobuo Cohomology of classifying spaces of certain associative H-spaces. J. Math. Kyoto Univ. 15 (1975), no. 3, 607–617.

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Thanks Mark! Do we have any results for spin group: $H^*(BSpin(n),\mathbb{Z}) =$? (say for $n=10$). –  Xiao-Gang Wen Feb 18 '13 at 13:25
You're welcome! For the Spin case take a look at R. Stong's "Notes on cobordism theory", around page 290. You can certainly piece together the additive structure from what's there, I'm not sure about the multiplicative structure with integral coefficients. –  Mark Grant Feb 18 '13 at 13:55

When $n$ is a prime, the additive structure of $H^*(BPU_n, \mathbb Z)$ has been computed independently in Kameko, Masaki; Yagita, Nobuaki, The Brown-Peterson cohomology of the classifying spaces of the projective unitary groups ${\rm PU}(p)$ and exceptional Lie groups. Trans. Amer. Math. Soc. 360 (2008), no. 5, 2265–2284 and in Vistoli, Angelo, On the cohomology and the Chow ring of the classifying space of ${\rm PGL}_p$. J. Reine Angew. Math. 610 (2007), 181–227. For $n = 3$, the second paper contains a computation of the multiplicative structure.

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