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In probability theory, the law of rare events or Poisson limit theorem states that the Poisson distribution may be used as an approximation to the binomial distribution, under certain conditions. [1] The theorem was named after Siméon Denis Poisson (1781–1840).



Let   be a sequence of real numbers in   such that the sequence   converges to a finite limit  . Then:








This leaves


Alternative ProofEdit

Using Stirling's approximation, we can write:


Letting   and  :


As  ,   so:


Ordinary Generating FunctionsEdit

It is also possible to demonstrate the theorem through the use of Ordinary Generating Functions of the binomial distribution:


by virtue of the Binomial Theorem. Taking the limit   while keeping the product   constant, we find


which is the OGF for the Poisson distribution. (The second equality holds due to the definition of the Exponential function.)

See alsoEdit


  1. ^ Papoulis, Pillai, Probability, Random Variables, and Stochastic Processes, 4th Edition