global shape of the universe. The problem of testing this kind of hypothesis can be solved by the Local Noise Correlations (LNC) method proposed in [@BerGo]. The problem of generating such kind of hypotheses seems to be much harder, although current efforts are being done to find multiple images of our Galaxy [@Galaxy], clusters of galaxies [@Rou] and radio-loud AGNs [@RMBS].

To close this paper, let us stress that there have only been considered here models in which the topological images are related by the generators of the covering groups of the corresponding manifolds. This needs not be the case, for one could also consider other isometries (compositions of the generators) as being the responsible for the multiple images. Thus the list of possible models presented here is not exhaustive.

Acknowledgments {#acknowledgments .unnumbered}
===============

I would like to thank CLAF/CNPq and FAPESP (contract 02/12328-6) for the grants under which this work was carried out, and to the CBPF for kind hospitality. I also thank to Bruno Mota, Marcelo Rebouças and Armando Bernui for critical reading of previous versions of this work and for their valuable suggestions.

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