(adsbygoogle = window.adsbygoogle || []).push({}); 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 (>).

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

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