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

What is a compact space?

  1. Jan 30, 2007 #1
    According to definition, a compact set is one where every open cover has a finite sub-cover.
    So let say I have C1, which is an open cover, I have C2 subset of C1 which is also an open cover. But C2 is finite.
    But since C2 is an open cover then there is a finite subcover C3 which is subset of C2.
    And so on and so forth , we will definitely end up with Cz which may only have one element. Then there will be no more subset of Cz, then how can there be any more subcover?
    Isn't there a contradiction?
  2. jcsd
  3. Jan 30, 2007 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, but C3 can be the same as C2, it doesn't have to have smaller cardinality.
  4. Jan 30, 2007 #3
    I see. So you mean subset but not necessarily proper subset suffices?
    I have gotten confused by the symbol.


    Does it mean subset or proper subset?
    Last edited: Jan 30, 2007
  5. Jan 30, 2007 #4
    This was exactly the same difficulty that I had with the notion of a compact set. I originally though a compact set meant a single point! But it doesn't.

    In modern notation, it would mean a proper subset, but much of the older texts and definitions of topology use it in its more ambiguous meaning as simply a "subset", proper or equal.

    A more straightforward definition of compactness is simply to say that:
    A compact set is one where every open cover is either finite or has a finite sub-cover.

    A compact set is an extention of the idea of a closed bounded set in spaces where neither close nor bounded makes much sense. In euclidean space, or spaces isomorphic to some euclidean space, compactness is equivilant to being closed and bounded.

    I think the old notion of allowing a subset to be less than or equal to is a notion that really should be retired. It's confusing, especially in cases like this. It's continued survival is probably for reasons of ostentation rather than clarity.
  6. Jan 30, 2007 #5
    "A compact set is one where every open cover is either finite or has a finite sub-cover." - Good definition!

    That is one cursed problem in math for the new-comer.
    The other one is the


    Different authors define it differently.
    I mean maths is supposed to be clear, logical and universal, how can that happen?
    Last edited: Jan 30, 2007
  7. Jan 31, 2007 #6
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook