Is there any explicit way to compute the cohomology groups $H^{4}( \mathbb{Z}/3\mathbb{Z}\times \mathbb{Z}/3\mathbb{Z},\mathbb{C}^{\times})$?. If it is nontrivial then how to produce a non trivial element in this group.
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yields that your group is isomorphic to $\mu_3^2$, and that the $H^2$ is isomorphic to $\mu_3$. So to cosntruct a non trivial element of $H^4$, I would try first to take the non-trivial element of $H^2$ and take the cup-product with itself...
GreginGre
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