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Combinatorics Help! Tricky problem here!

  1. Sep 26, 2012 #1
    1. The problem statement, all variables and given/known data

    Consider f:[7]→[9]

    How many functions are there in which f^-1 is not a function?

    2. Relevant equations

    Don't seem to fine one.

    3. The attempt at a solution

    Try drawing the function diagram, given f:[7]→[9]. Then, count the total number of functions. Take too long to do so, so I quit. I just know that the inverse function must have at least an element from dom(f) to a specific element or more. For instance, f(1) = f(2) = f(3).

    ** EDIT **

    Nevermind. I got the answer.
     
    Last edited: Sep 26, 2012
  2. jcsd
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