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Projection onto Linearly Dependent Vectors

Now consider another example:

\sv_0 &\isdef & [1,1], \\
\sv_1 &\isdef & [-1,-1].

The projections of $ x=[x_0,x_1]$ onto these vectors are

{\bf P}_{\sv_0}(x) &=& \frac{x_0 + x_1}{2}\sv_0, \\
{\bf P}_{\sv_1}(x) &=& -\frac{x_0 + x_1}{2}\sv_1.

The sum of the projections is

{\bf P}_{\sv_0}(x) + {\bf P}_{\sv_1}(x) &=&
\frac{x_0 + x_1}...
... + x_1}{2} (-1,-1) \\
&=& \left(x_0+x_1,x_0+x_1\right) \neq x.

Something went wrong, but what? It turns out that a set of $ N$ vectors can be used to reconstruct an arbitrary vector in $ {\bf C}^N$ from its projections only if they are linearly independent. In general, a set of vectors is linearly independent if none of them can be expressed as a linear combination of the others in the set. What this means intuitively is that they must ``point in different directions'' in $ N$-space. In this example $ s_1 = - s_0$ so that they lie along the same line in $ 2$-space. As a result, they are linearly dependent: one is a linear combination of the other ( $ s_1 = (-1)s_0$).

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