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Homework Help: Defining functions from sets

  1. Apr 2, 2009 #1
    Let A = {{1,2},{1,3},{2},{2,5},{3,4,5}}. Define f : A-->N by f(a)=the sum of the elements of a (for all a that are a member of A).

    I am just not sure how to define a function from a set. I have been really sick and haven't been able to make it to class :( Can someone point me in the right direction?
     
  2. jcsd
  3. Apr 2, 2009 #2
    So I have been reading some other posts and I think I understand it a bit more. Tell me if this would be right:

    f( {2} ) = 2,
    f( {1,2} ) = 1 + 2 = 3,
    f( {1,3} ) = 1 + 3 = 4,
    f( {2,5} ) = 2 + 5 = 7,
    f( {3,4,5} ) = 3 + 4 + 5 = 12.

    And so if I were to find the range of f it would be f = {2, 3, 4, 7, 12}??
     
  4. Apr 2, 2009 #3

    HallsofIvy

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    Science Advisor

    Yes, that is exactly what "the sum of the elements of a" means! And since, as usual, "range of f" means "set of all values of f(a)", the range is {2, 3, 4, 7, 12} as you say.
     
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