configuration one has $$\label{ExampleSecondG6}
|c| = 791.6h^{-1} \, Mpc \quad\mbox{,}\quad z = -5.8h^{-1} \, 
Mpc \quad\mbox{,}\quad |b| \leq 834.1h^{-1} \, Mpc \; ,$$ the last inequality being obtained from the last equation in (\[ModelSecondG6\]). It is illustrative to give two specific examples as done with the ${\mathcal{G}_2}$ models.

1.  First consider the case when $|b|$ is slightly lower than its maximum value allowed by (\[ExampleSecondG6\]), say $|b| = 
    834h^{-1} \, Mpc$, then one has $x = \pm 7h^{-1} \, Mpc$.

2.  Second consider the symmetric case when $|b| = |c| = 
    791.6h^{-1} \, Mpc$. In this case $x = \pm 131.5h^{-1} \, Mpc$.

The case of three triples of images {#Decide}
===================================

In this section it is shown that the discovery of two additional triples of clusters of galaxies close to $\{C_0,C_1,C_2\}$ would allow the determination of the topology of the universe, and in most cases the determination of its size. Let us denote by $\{D_0,D_1,D_2\}$ and $\{E_0,E_1,E_2\}$ these two additional triples of topological images. Mathematically, to characterize the *closeness* relation between two triples $\{C_i\}$ and $\{D_i\}$ it suffices the lengths of the geodesic segments $\overline{C_iD_i}$ ($i=0,1,2$) to be the same and smaller than the injectivity radius. Observationally, it is 