  We study the topological signature of euclidean isometries in pair
separations histograms (PSH) and elucidate some unsettled issues regarding
distance correlations between cosmic sources in cosmic crystallography.
Reducing the noise of individual PSH's using mean pair separations histograms
we show how to distinguish between topological and statistical spikes. We
report results of simulations that evince that topological spikes are not
enough to distinguish between manifolds with the same set of Clifford
translations in their covering groups, and that they are not the only signature
of topology in PSH's corresponding to euclidean small universes. We also show
how to evince the topological signature due to non-translational isometries.
