the multiple images fit with the first configuration. On the other hand, with the second configuration one can determine the vectors $b$ and $c$, and thus the direction of $a$, but it is impossible to determine the length $|a|$, as could have been anticipated from eqs. (\[EqSecondG6\]). However, in this latter case, one can design effective search procedures to look for multiple images due to the isometries $\alpha$ and $\alpha^{-1}$, thus providing at least robust constraints for the parameter $|a|$ (see [@BerGo]).

Discussion and Further Remarks {#Concl}
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The work presented in this paper originated with the following problem in the context of cosmological models with flat spatial sections: Suppose we have identified three clusters of galaxies as being different topological images of the same object. How do these multiple images constrain the possible models for the shape of our Universe? A natural extension of this work would be the study of this problem in the context of universes with non–flat spatial sections, specifically those with positive curvature (since multiply connected spaces of negative curvature are very unlikely to have a detectable topology [@Detect]).

It has been shown here that one can accommodate any of the six classes of compact 