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Why do we associate numbers with a line?

  1. Jan 8, 2012 #1
    Why do we associate numbers with a line?
    It just dawned on me that I was never told why and that all the math I have studied so far assumed that numbers could be placed on a number line =/. Questioning this seems odd to me but I would like to exactly know why. I can't think of a reason.

    Also the number zero... is the only point on the number line that is neither positive nor negative. Does it only exist because in order for us to place numbers on a number line we have to have some point to join negative numbers and positive numbers? Without it would be kind of hard to place both positive numbers and negative numbers on a line wouldn't? I've been thinking about it some and zero is... smaller than the most smallest negative number and smaller than the most smallest positive number... um also we assume that zero is indeed the only point on the number line were positive numbers and negative numbers meet and that there is no number which exists that is larger than the largest positive number +∞ and larger than the most largest negative number -∞? But yet we have zero as this point smaller than the smallest positive and negative numbers? Why can't we have both and have the number line behave like a line until you got to a point were numbers got so large that the number line bend and curved back to the negative numbers.... or just have zero be the point joining -∞ and +∞ instead of it's location between the two smallest numbers and have a number line like this...?

    what if there really were to zeros on the number line, one joining the smallest positive and negative numbers and one joining the largest positive and negative numbers... then when you divide by zero you can't because you are trying to divide by two different numbers or something...
  2. jcsd
  3. Jan 8, 2012 #2
  4. Jan 14, 2012 #3
    We associate numbers with the plane, the complex plane.
  5. Jan 14, 2012 #4


    Staff: Mentor

    We associate pairs of numbers with the plane.
  6. Jan 14, 2012 #5


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    Homework Helper

    That's an extension. The number line is just that - a line.

    By the way, you missed a value of pi's decimal expansion in your name

  7. Jan 14, 2012 #6
    Zero is larger than any negative number.
  8. Jan 26, 2012 #7
    I think 3.14159253589 means that only real numbers are associated with a line?
  9. Jan 26, 2012 #8
    Because they are ordered, and integers are "well ordered". You will learn more about ordering and largest this and smallest that in analysis.
  10. Jan 26, 2012 #9
    Positive integers are well-ordered.
  11. Jan 26, 2012 #10
    Good call. Although they can be well-ordered.
  12. Jan 26, 2012 #11
    The reals are associated with a line because that is the simplest geometry that they can be embedded in.
    An extension of the reals is the projective real line, which adds the value that you talk about, making the full set like a circle.
    Extending that to complex numbers associates with a sphere, the Reimann sphere.

    (I personally think you can take the extension further to define a filled in sphere -a ball- but this forum isn't for speculation).
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