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Just a Yes or No Question

  1. Sep 10, 2008 #1
    Does ]-∞, ∞[ (can also be written as -∞<x<∞) mean all real numbers?
  2. jcsd
  3. Sep 10, 2008 #2
    ]-∞, ∞[

    Never seen it written like that I would be more inclined to write :
    [-∞, ∞] or more commonly me thinks (-∞, ∞) - to show that this is open

    When you write -∞<x<∞ I guess the most common assumption is that x is contained in the reals but without context it really isn't smart to assume anything. Usually someone will specifically state x contained in the reals and -∞<x<∞ - or x contained in the integers and -∞<x<∞ or the set that x belongs to will be clear from the context.

    (Sorry for not responding with yes or no but hopefully you understand why I could not)
  4. Sep 10, 2008 #3
    If the brackets are facing outwards ][ then they're supposed to mean the points are excluded.
    If the brackets are facing inwards [] then they're supposed to mean the points are included.
    At least, that's what I was told, and that's what I read in my textbook.

    Anyway, yes, I see your point. Another number with the answer of "all real numbers" simply showed the symbol for it, so I'm pretty wary of assuming that -∞<x<∞ means "all real numbers" as well.

    Thank you for your input though. :)
  5. Sep 10, 2008 #4
    I think the ] [ notation is used in other countries other than US.
  6. Sep 10, 2008 #5


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    In general, I'd say yes. But, it's normally easier to write something like [itex]x\in\mathbb{R}[/itex]
  7. Sep 10, 2008 #6
    Must be, in the US we typically use () to show open, and [] to show closed.
  8. Sep 10, 2008 #7
    yes that's exactly what that notation means and pretty much the only legit use of the symbol ∞
  9. Sep 11, 2008 #8
    Aaaah, thanks. I asked my math teacher before I read the last few posts, and confirmed that the answer is yes as well. :)
  10. Sep 11, 2008 #9
    You say

    Does ]-∞, ∞[ (can also be written as -∞<x<∞) mean all real numbers?

    first of all there is a difference between ]-∞, ∞[ and -∞<x<∞, the first is all the real numbers, the second is the same as [itex]x \in ]-∞, ∞[ [/itex]. You are right that ]-∞, ∞[ is all the real numbers, but an interval is defined from the real numbers, so that notation is rarely used.
  11. Sep 11, 2008 #10
    the itex tags are misrepresenting your point. I view it as x in ] -8743, 8743 [

    But what you actially wrote was x \in ]-∞, ∞ [ with itex tags.
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