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Topology and order type problem

  1. Jul 13, 2010 #1
    1. The problem statement, all variables and given/known data
    Both {1,2}x Z+ and Z+ x {1,2} are well-ordered in the dictionary order. Are they of the same order type? Why or why not?


    2. Relevant equations
    To be of the same order type, we must be able to construct a bijection that preserves order, that is, x<y => f(x)<f(y). well-ordered means that every nonempty subset has a minimal element.


    3. The attempt at a solution
    I know I need to find a bijection between the two, but I can't seem to think of one. I can get a bijection from Z+ x {1,2} to Z+, but I can't find one from the other one to Z+
     
  2. jcsd
  3. Jul 13, 2010 #2
    Re: Topology

    oh yeah- Z+ is the set of positive integers
     
  4. Jul 13, 2010 #3

    Hurkyl

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    Re: Topology

    Are you sure?
     
  5. Jul 13, 2010 #4
    Re: Topology

    well, I don't know how I can prove that there is no bijection. Again, I know I can find a bijection from Z+ to {1,2}xZ+, but listing the other set in order would entail TWO infinities. It would be (1,1),(1,2),(1,3)... (2,1),(2,2),(2,3)... is that enough to say that Z+ x {1,2} cannot be bijected into Z+ and therefore there is no bijection between them?
     
  6. Jul 13, 2010 #5

    Hurkyl

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    Re: Topology

    That's the basic idea.

    However, given that you're just learning this stuff, your teacher probably expects you to both provide the basic idea and fill in the details to turn it into a proof.

    e.g. why would having "two infinities" make a difference? Are you sure Z+ doesn't have "two infinities"?
     
  7. Jul 13, 2010 #6
    Re: Topology

    okay- thanks!!
     
  8. Jul 13, 2010 #7

    Hurkyl

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    Re: Topology

    Oh, and for the record -- you want the term "order isomorphism", not "bijection". While every order isomorphism is a bijection, not all bijections are order isomorphisms.
     
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