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\item $f_p$ is the fraction of stars that have planets   \item $f_l$ is the fraction of planets where life actually begins   \item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_i$ $f_c$  is the fraction of intelligent civilization that decide to communicate \end{itemize}  Finally $L$ is the longevity of a communicative civilization (in years). In the case of humankind L is, so far, just a few decades.