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Discrete Mathematics help

  1. Nov 18, 2007 #1
    1. The problem statement, all variables and given/known data
    Find the range of the function g: ZxZ --> ZxZ given by g(m,n)=(m-n,m+n). Hints: First recall that if f: A ---> B then Range (f)={b e B such that there exists an A in A with b=f(a). Second, if you claim that some set C is the range, then you must show that i) C is a subset Range(f) and ii) range (f) is a subset of C. Both i and ii are required to conclude that C=range(f).


    2. Relevant equations
    none


    3. The attempt at a solution
    I am think that the range is all integers from -infinity to +infinity.

    I know that:
    * ZxZ denotes a set of ordered pairs.
    * m,n exist in Z
    * range (g) exists in Z

    I don't know where to go with this!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 18, 2007 #2

    Dick

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    The range is all pairs (p,q) such that (p,q)=(m+n,m-n) for integers m and n. Solve for m and n in terms of p and q. Now rethink your guess that the range is all integer pairs. E.g. is (1,2) in the range?
     
  4. Nov 18, 2007 #3
    So we figured out that p=q. So our new guess for the range is all integers such that p=q for the ordered pair (p,q). If we say that, is that enough to justify our answer?
     
  5. Nov 18, 2007 #4

    Dick

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    How did you figure out p=q? p=m+n, q=m-n for the domain value (m,n) and ask yourself when m and n are both integers. That's two equations in two unknowns. Solve them. Your new guess is worse than the last.
     
  6. Nov 18, 2007 #5

    Dick

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    Actually, let's go back. You never answered my question. Is (1,2) in the range? If you answer that it will give you a big clue in a non-abstract sense.
     
  7. Nov 18, 2007 #6
    (1,2) is not in the range. When substituted in the equations for p and q the results for m and n are not integers.
    After looking at your first response to our terrible answer, we looked that problem and decided that the range is (p,q) when p and q are any even integers.
    **I am working on this a group of people and the more we talk about it, the more confused we get.**
     
  8. Nov 19, 2007 #7

    Dick

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    If you solved those equations you should have gotten m=(p+q)/2 and n=(p-q)/2. Now think about pairs (p,q) where both p and q are odd.
     
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