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Suppose $X_1$ and $X_2$ are two nice metric spaces, e.g. two Riemannian manifolds, and let $G_i=Isom(X_i)$. Then $G_1\times G_2\subset Isom(X_1\times X_2)$.

Suppose $X_1\times X_2$ is not compact and let $\Gamma\subset G_1\times G_2$ act cocompactly on $X_1\times X_2$. In general the projections of $\Gamma$ to $G_1$ and $G_2$ are not discrete.

Example: Let $X_1=S^1$, $X_2=\mathbb R$, and $f=(\alpha,\tau)$ where $\alpha$ is an irrational rotation an $\tau(x)=x+1$. The group generated by $f$ is discrete but its projection to $G_1$ is not discrete.

Is there a characterizatrion (or a sufficient condition) of the situation where both projections of $\Gamma$ to $G_1$ and $G_2$ are discrete?

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  • $\begingroup$ $\Gamma$ could be of the form $\Gamma _1\times \Gamma _2$, (up to finite index). $\endgroup$ – Venkataramana Oct 21 '13 at 15:52
  • $\begingroup$ I think in the Riemannian case the answer is essentially the same as in the symmetric case: $\Gamma$ is virtually a product iff the projection to each factor is discrete. Is it what you are after? $\endgroup$ – Misha Oct 24 '13 at 5:40
  • $\begingroup$ yes, thank you. Also thank to 39082 for the reference in the case of trees. $\endgroup$ – user126154 Oct 24 '13 at 9:56
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This is presumably a hard question, see e.g. http://download.springer.com/static/pdf/747/art%253A10.1007%252FBF02698916.pdf?auth66=1382759591_9b0973395ce39560e6d6963480bf665c&ext=.pdf

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