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Homework Help: Inverse mapping of a singleton set

  1. Sep 13, 2008 #1
    1. The problem statement, all variables and given/known data
    For X= NxN, Y=N, define the mapping phi: X-->Y as phi(x,y)=x+y. Find the inverse image of phi-inverse (5) of the singleton set {5}. If n: X-->Y is the product operation n(x,y)=xy, find n-inverse (4).

    3. The attempt at a solution
    I'm not even really sure what the question is asking. If the set is a singleton set, does that mean it sets 5 to itself? So the inverse image would be 5...?

    As for the product operation, I don't know how to approach it.
  2. jcsd
  3. Sep 13, 2008 #2


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    The inverse image of {5} is the set of all things that map to 5 under phi. In the first case all (x,y) in NxN such that x+y=5 and in the second case x*y=4. N is the natural numbers, right?
  4. Sep 13, 2008 #3
    N is the natural numbers.

    So for x+y=5, the inverse image is either x=0,y=5, or x=5,y=0?

    And the inverse image for xy=4 is either x=1,y=4, or x=4,y=1?

    Do I have to show a proof for it though?
  5. Sep 13, 2008 #4


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    How about x=1,y=4. Doesn't that work in the first case? And is 0 a natural number? I don't think you have all the solutions for the second case either. I'd just start by listing the possibilities. It doesn't seem to me like it's necessary to 'prove' it.
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