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Let's define $n=\frac{N}/{V}$ $n=\frac{N}{V}$  the number density of communicative civilizations in the Galaxy, where $N$ is the total number of such civilizations populating the Galaxy and $V$ is the total galactic volume. Assuming an homogeneous distribution across the Galaxy, the average spacial separation between two neighbour communicative civilization is $\sim 1/n^{1/3}$. $r\sim \frac{1}{n^{1/3}}$.