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Homework Help: Any point in [\tex]mathbb{R}^{3}[\tex]

  1. Mar 10, 2010 #1
    Any point in 3-dimensional space

    Is is possible to write any point [tex] {\bf x}\in \mathbb{R}^{3}[/tex] as a linear combination of vectors [tex]{\bf v_{j}}[/tex] inside the unit ball over [tex]\mathbb{R}[/tex] given that

    [tex]\sum_{j}c_{j}{\bf v_{j}}[/tex]

    can be made arbitrarily small?

    My approach was letting

    [tex] \parallel{\bf x}\parallel \leq \bigg{\parallel} \sum_{j}c_{j}{\bf v_{j}}\bigg\parallel (*)[/tex]

    Then applying the triangle inequality to the two terms then (*), the triangle inequality again to the linear combination, [tex] \parallel v_{j}\parallel<1 [/tex] for all [tex]n[/tex] then the Cauchy's inequality to obtain

    [tex] \parallel{\bf x} - \sum_{j}c_{j}{\bf v_{j}}\bigg\parallel [/tex]

    [tex] \leq 4n\sum_{j}|c_{j}|^{2} [/tex]

    where n is the number of terms in the linear combination, which is finite.

    Therefore by letting [tex] \sum_{j}|c_{j}|^{2} \rightarrow 0[/tex],

    we get

    [tex]\parallel{\bf x}- \sum_{j}c_{j}{\bf v_{j}}\bigg\parallel \leq 0 [/tex]

    Thus deducing

    [tex] {\bf x} = \sum_{j}c_{j}{\bf v_{j}} [/tex]

    I am not sure if assuming (*) is the correct way of doing this since we miss out on the other possibility (>).
    Last edited: Mar 10, 2010
  2. jcsd
  3. Mar 11, 2010 #2


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    Re: Any point in 3-dimensional space

    I'm guessing you haven't told us everything. Let

    [tex]v_1 = \frac x {2||x||} \hbox{ so } ||v_1|| = \frac 1 2[/tex]

    Let [itex]c = 2||x||[/itex]. Then and [itex]x = cv_1[/itex].
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