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Homework Help: Concerning Subspaces of Infinite Dimensional Vector Spaces

  1. Apr 19, 2012 #1
    I have a question concerning subspaces of infinite dimensional vector spaces. Specifically given any infinite dimensional vector space V, how might one construct an infinite decreasing chain of subspaces?

    That is:

    V=V0[itex]\supseteq[/itex]V1[itex]\supseteq[/itex]... , where each Vi is properly contained in Vi-1.

    I know such chains must exist and I suspect that they should be easily constructed, however I am not familiar enough with infinite dimensional vector spaces to be confident with my attempts thus far.

    Thanks for any help given!
  2. jcsd
  3. Apr 19, 2012 #2
    Select a basis for the vector space. This should be finite, so we can select a countable chain [itex]\{e_1,e_2,e_3,e_4,...\}[/itex] of basis elements.


    [tex]span\{e_1\}\subseteq span\{e_1,e_2\}\subseteq span \{e_1,e_2,e_3\}\subseteq ...[/tex]

    is an infinite chain of subspaces.
  4. Apr 19, 2012 #3

    So I suppose that with your notation:

    [tex]V = span\{e_1,e_2,...\}\supseteq span\{e_2,e_3,...\}\supseteq...[/tex] will give me the infinite decreasing chain as desired.

    Thanks again for your help!

    EDIT: On second thought, can I be assured that such a chain will not stabilize? It is clear in your construction for building increasing chains but it seems less intuitive for building a decreasing chain in such a way.
    Last edited: Apr 19, 2012
  5. Apr 20, 2012 #4
    If the e_i are components in a basis, then e_n is not in the span of {e_(n+1), e_(n+2),... }, so each inclusion will be proper.
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