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

Intersection of two open sets

  1. Aug 23, 2007 #1
    [SOLVED] intersection of two open sets

    does anyone know how to prove the intersection of two open sets is open?
     
  2. jcsd
  3. Aug 23, 2007 #2

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, do you? What is the definition of an open set?
     
  4. Aug 23, 2007 #3
    (....) ∩ (....) = (.......)?

    lol
     
  5. Aug 24, 2007 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    There's nothing to prove about it. The axioms of a topological space (topology over a set) claim that the intersection of an arbitrary but finite # of open sets is open.
     
    Last edited: Aug 24, 2007
  6. Aug 24, 2007 #5

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    One presumes the OP is talking about the usual notion of open and closed in R. There is something to prove then if one wishes to show this is actually a model of the axioms of a topological space, dexter.

    But, this does show the place to start is with the OPs definition of open (or for that matter, in what space are we even talking? R, R^n?)
     
  7. Aug 24, 2007 #6
    the open sets are in R, but i need to prove that the intersection of just two open sets is open. once i have that, proving the intersection of a finite number of open sets is easy. i'm trying to use an open ball in the proof.
    i'm at a loss.
     
  8. Aug 24, 2007 #7

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    Let's look at the problem. Let's take (a,b) and (c,d) as the open intervals. What is the intersection of those two sets? There are only a few cases to consider - they're disjoint, one is a subset of the other, and they overlap.

    If you want to be fancier, and allow for arbitrary open sets, then what does it mean for X to be open? It means for each x in X there is an e depending on x such that (x-e,x+e) is wholly contained in X. Now suppose that x is in the intersection of X and Y. That means what? That there is an e such that (x-e,x+e) lies wholly in X, and a d so that (x-d,x+d) lies wholly in Y. Can you now see how to find an f so that (x-f,x+f) lies wholly in X and wholly in Y (i.e. in the intersection thus proving that the intersection is open)?
     
  9. Aug 24, 2007 #8
    i have been able to prove that there is an e such that (x-e,x+e) is wholly in X and there is an f such that (y-f,y+f) is wholly in Y, i'm having a problem proving that there exists a g such that (z-g,z+g) exists only in the intersection of X and Y. i can't figure out what g should equal to make sure that the it stays in the intersection.
     
  10. Aug 24, 2007 #9

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    You can't? Well, what if I said x=0 which I may as well do. Then what if e=1 and g=2? Can you really not think of an open interval contained in both (-1,1) and (-2,2)?

    And why do you have x,y, and z? You only have to suppose you are looking at one point x lying in both X and Y.
     
  11. Aug 24, 2007 #10
    i am having a major brain fart. ok, so if i'm only looking at one point x that is an element of the intersection of X and Y, and e>0.
    then the i have to consider the cases:
    x is an element in X or x is an element in Y and they do not over lap, then the intersection is obviously open because the intersection is the empy set
    so the case i need is when X n Y does not equal the empty set.
    got it and i know how to prove an open ball is a subset of X n Y, i just can't figure out what i need my e=?
     
  12. Aug 24, 2007 #11

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    If x is in open set A, there exist [itex]\delta_1> 0[/itex] such that [itex](x-\delta_1,x+\delta_1)[/itex] is a subset of A. If x is in open set B, thre exist exist [itex]\delta_2> 0[/itex] such that [itex](x-\delta_2,x+\delta_2)[/itex] is a subset of B. Now, how can you find a [itex]\delta> 0[/itex] so that both of those are true?
     
  13. Aug 24, 2007 #12
    i'm trying to use the definition of intersection in my proof, but i can't seem to get it right. i can prove the x is an element in X or Y, but it's the and that's tripping me up.
     
  14. Aug 24, 2007 #13

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    I'm goint to repeat my question. Are you saying that you cannot see an open interval that is contained in (-1,1) and (-2,2).


    Can you see one that is a subset of (-10,10) and (-0.1, 0.1)?
     
  15. Aug 24, 2007 #14
    see yes, prove no
     
  16. Aug 24, 2007 #15

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    What open set are you thinking of? Write an open subset of (-1,1) and (-2,2) out for me.
     
  17. Aug 24, 2007 #16
    i have to prove it for any X and Y. i has to be abstract, all theory. which my prof told me i have to follow the logic and i'm stuck.
     
  18. Aug 24, 2007 #17

    EnumaElish

    User Avatar
    Science Advisor
    Homework Helper

    You'll never get to abstract logic if you cannot solve a simple, concrete example.

    If you want to be unstuck I suggest you try to work through a simple example like the one above.
     
    Last edited: Aug 24, 2007
  19. Aug 24, 2007 #18
    it's the number line with shaded between -1 and 1 with circles at the end to indicate that the end points are not included in the set

    i'm thinking more of a Venn Diagram for a picture.
     
  20. Aug 24, 2007 #19

    EnumaElish

    User Avatar
    Science Advisor
    Homework Helper

    Stay with the example. What is your e, f and g for x = 0?
     
    Last edited: Aug 24, 2007
  21. Aug 24, 2007 #20
    i have e>0 and f>0, and i tried g=min{e,f} but that doesn't work because it doesn't guarantee that g will be small enough to only exist in the intersection of X and Y
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Intersection of two open sets
Loading...