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Homework Help: Nice ques to solve

  1. Jul 29, 2011 #1
    Nice ques to solve !!!

    1. The problem statement, all variables and given/known data

    Let set 'A' have 'n' number of elements and let set 'B' have 'm' number of elements and let their's a defined function f:A→B. Determine the number of possible 'onto' functions that are possible and valid..!!..prove it by mapping of elements from 'A' to 'B'..!!

    2. The attempt at a solution

    According to my logic there are mP1 + mP2 + mP3 +..... + mPn Permutations..
    is my answer correct....and i could understand that Only if m=n, we will have mPn onto functions. If n<m, none of the functions would be onto...is my statement correct for n>m...and also gimme other logic or methods..!!
  2. jcsd
  3. Aug 2, 2011 #2


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    Gold Member

    Re: Nice ques to solve !!!

    To be a little more clear would you be able to write the question word for word as it appears in your homework?

    Are you asking "How many surjective functions exist from a domain of m elements to a co-domain of n elements?"? I believe that there is a very handy method that gives this answer with one equation involing a 0 to n summation, have you covered any such equations?

    Edit, looking back it seems that you are trying to derive that equation.

    Think about the total number of functions. Can you think of a way to find the number of non-surjective functions?

    Hint: A non surjective function going to the co-domain B can be discribed as a surjective function going to a smaller co-domain.
    Last edited: Aug 2, 2011
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