Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Probability (probability mass function,pmf)

  1. Dec 15, 2009 #1
    If X has the probability mass function f(x) = k / x! (x=0,1,2,...),
    what is the value of k?
  2. jcsd
  3. Dec 15, 2009 #2


    User Avatar
    Science Advisor

    I think you mean "probability density function".

    The total probability must be 1.
    [itex]\sum_{x=0}^\infty k/x!= k\sum_{x=0}^\infty 1/x!= 1[/itex]

    Do you know what that sum is?
  4. Dec 15, 2009 #3
    i dont know.. is it infinity?
  5. Dec 15, 2009 #4


    User Avatar
    Homework Helper

    For discrete distributions these are usually called mass functions - density is reserved for continuous distributions.

    naspek, HallsofIvy is asking whether you can identify what the sum

    \sum_{x=0}^\infty \frac 1 {x!}

    equals? This problem should remind you of a commonly used distribution.
  6. Dec 15, 2009 #5


    User Avatar
    Science Advisor

    You probably have not taken calculus yet, but if you had you would have learned that
    [tex]e^x= \sum_{n=0}^\infty \frac{x^n}{n!}[/tex].

    Do you know what an "exponential probability distribution" is?
    Last edited by a moderator: Dec 16, 2009
  7. Dec 15, 2009 #6
    ok.. now i understand already... thanks guys!! =)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook