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Finite Intersection Property Question

  1. Jun 23, 2010 #1
    I'm having a little trouble distinguishing the line between what the f.i.p implies and what it does not.

    **EDIT2** Hopefully this will make things more clear

    What I'm really interested in is a formal definition of the f.i.p regardless of the set in question or the field.

    Given the sequence [itex] \{a_n\}_{n \in \mathbb{N}} [/itex] in some field [itex] \mathbb{K} [/itex], is having the f.i.p equivalent to saying [itex] (\forall n \in \mathbb{N})(\exists x \in \mathbb{K})(x \in \cap_{i=1}^n a_n) [/itex]

    The question about the sequence was that I was afraid (even though it seems pretty clear to me that this should follow from the previous statement) that it's construction assumed that the intersection of the entire set was non-empty as the sequence itself is infinite and says for all [itex] n \in \mathbb{N} [/itex] pick an element from the nth intersection.

    My question about the axiom of choice, was that, if I'm just given a sequence of nonempty sets (assuming no other properties like order complete) and I just say form a new sequence by picking some random element from each set in the old sequence, does this require the Axiom of Choice?



    Specifically, if we are working in R and [itex] \{[a_i, b_i]\}_{i \in \mathbb{N}} [/itex] satisfies the finite intersection property, then does that imply that [itex] (\forall n \in \mathbb{N})(\exists x \in \mathbb{R})(x \in \cap_{i=1}^n [a_i, b_i]) [/itex], and, if it does, then can we use the axiom of choice (I believe this is needed...) to define a sequence [itex] \{c_n\}_{n \in \mathbb{N}} [/itex] where [itex] c_n \in \cap_{i=1}^n [a_i, b_i] [/itex]?


    **EDIT** I stated R above to make things simple, but it may help to know that in reality I'm trying to prove that sequential (Cauchy) completeness in a totally ordered Archimedean field implies that every sequence of bounded closed interval with the finite intersection property has a nonempty intersection.
    Last edited: Jun 23, 2010
  2. jcsd
  3. Jun 23, 2010 #2


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    [noparse] Regarding the LaTeX... Start with [tex] and end with [/tex]. Alternatively, use itex instead of tex. itex is supposed to look better if the math expression appears in the middle of a line of text, but it doesn't always. itex also cuts off the top of some large symbols. You can (and should) preview before you post, but there's a bug that makes old latex images show up in all previews except the first. When that happens, just refresh and resend.[/noparse]
  4. Jun 23, 2010 #3
    Thanks for the LaTeX tip, I changed it
  5. Jun 24, 2010 #4
    F.I.P. is a property of a space that whenever a family of closed subsets is such that any finite subcollection has a non-empty intersection, so does the entire family.

    F.I.P. is related to the Axiom of Choice by Tychonoff's theorem which states that the Cartesian product of compact topological spaces is compact.

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