Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Set Theory Sketches.

  1. Apr 2, 2012 #1
    So the book asks me to sketch out these graphs, and of course there are no examples. I was wondering how this is done.

    (a) [0,1] X [1, 2] // The X here stands for the Cartesian product.

    (b) ([0,1] U {2}) X [1,2] // How can I graph this? The U stands for Union and the X here stands for the Cartesian Product.

    (c) ([0,1] U {2}) X ([1,2] U {3}).// Again, the U stands for Union and the X here stands for the Cartesian Product.
  2. jcsd
  3. Apr 2, 2012 #2
    I imagine they mean rectangular areas in the [itex]\mathbb{R}^2[/itex] plane. For the (a) example, set one interval in the X-axis and the other in the Y-axis; this defines a rectagular area (the points which x is in [0,1] and which y is in [1,2]).
  4. Apr 2, 2012 #3
    Thanks, DODO that was what I suspected for part a, but parts b and c still mystify me.
  5. Apr 2, 2012 #4
    ([0,1] U {2})

    For example what would this look like?
  6. Apr 2, 2012 #5


    User Avatar
    Science Advisor

    Think of your Venn diagrams. If you have two sets A and B and you have A U B then you can have anything that is both A and B. If you have A [itex]\bigcap[/itex] B then you have any element that is both A and B. This is the definition of these two binary operators.

    Again if you get stick think of the Venn diagram graphically of what A [itex]\bigcup[/itex] B and A [itex]\bigcap[/itex] B in terms of pictures and then use that intuition to think of what the symbols mean.

    Simplify anything with [itex]\bigcup[/itex] and [itex]\bigcap[/itex] and then take the cartesian product after.
  7. Apr 2, 2012 #6
    So I uploaded my guess at it. What do you think? Is (b) correct? Does ([0,1] U {2}) simply become [0,1,2], which could be read as [0,2]?

    Attached Files:

  8. Apr 2, 2012 #7


    User Avatar
    Science Advisor

    You have to be careful with your notation.

    Usually when we want to describe a discrete (countable) set, we usually specify every element in the set and not just the first and last element. When we are talking about a continuous set like all real numbers from 0 to 2 inclusive then we say [0,2]. It is probably a better idea to specify your set as [0,1,2] just so there is no confusion. Your answer is right of course but your [0,2] to mean {0,1,2} is misleading: (also when we talk about sets we always put them in curly braces like {0,1,2}: [0,2] is usually used for describing intervals like 0 <= x <= 2)

    So for the sets {0,1} U {2} = {0,1,2} remember to use the curly braces just so no-one gets confused :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook