deletions | additions       diff --git a/Equation.tex b/Equation.tex  index 767a2a8..df1973b 100644  --- a/Equation.tex  +++ b/Equation.tex 
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\item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_c$ is the fraction of intelligent life-forms that decide to communicate  \end{itemize}  Finally $L$ is the longevity of a communicative civilization (in years). Humankind Humankind, for example,  has been "communicative" only for a few decades. Interestingly a detection (or even complete silence) would put strong constraints on $L$, in a sense telling something about the usual course of civilizations. In other words about the future of humankind. decades and we do not know for how long we will be.