Drake equation
The Drake equation was devised in an attempt to estimate the number of extraterrestrial alien species that could communicate with Humans. The first and more mathematical variation stated:

where:

  is the number of extraterrestrial civilizations in our galaxy with which we might expect to be able to communicate 

and

  is the rate of star formation in our galaxy 
  is the fraction of those stars which have planets 
  is average number of planets which can potentially support life per star that has planets 
  is the fraction of the above which actually go on to develop life 
  is the fraction of the above which actually go on to develop intelligent life 
  is the fraction of the above which are willing and able to communicate 
  is the expected lifetime of such a civilization 

The second variation stated:

In 1996, Rain Robinson had a poster of the two variations of the equation right behind her computer monitor in her office at the Griffith Observatory. ()

External link 
 

fr:Équation de Drake
Mathematics