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Linear Algebra

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262 Chapter 3. Maps Between Spaces<br />

(Verification that R 3 = M ⊕ N and R 3 = M ⊕ ˆ N is routine.) We will check<br />

that these projections are different by checking that they have different effects<br />

on this vector.<br />

⎛<br />

�v = ⎝ 2<br />

⎞<br />

2⎠<br />

5<br />

For the first one we find a basis for N<br />

⎛<br />

BN = 〈 ⎝ 0<br />

⎞<br />

0⎠〉<br />

1<br />

⌢<br />

and represent �v with respect to the concatenation BM BN .<br />

⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞<br />

2 1 0 0<br />

⎝2⎠<br />

=2· ⎝0⎠<br />

+1· ⎝2⎠<br />

+4· ⎝0⎠<br />

5 0 1 1<br />

The projection of �v into M along N is found by keeping the M part and dropping<br />

the N part.<br />

⎛ ⎞<br />

1<br />

⎛ ⎞<br />

0<br />

⎛ ⎞<br />

2<br />

projM,N(�v )=2· ⎝0⎠<br />

+1· ⎝2⎠<br />

= ⎝2⎠<br />

0 1 1<br />

For the other subspace ˆ N, this basis is natural.<br />

⎛ ⎞<br />

0<br />

BN ˆ = 〈 ⎝ 1 ⎠〉<br />

−2<br />

Representing �v with respect to the concatenation<br />

⎛ ⎞<br />

2<br />

⎛ ⎞<br />

1<br />

⎛ ⎞<br />

0<br />

⎛ ⎞<br />

0<br />

⎝2⎠<br />

=2· ⎝0⎠<br />

+(9/5) · ⎝2⎠<br />

− (8/5) · ⎝ 1 ⎠<br />

5 0<br />

1<br />

−2<br />

and then keeping only the M part gives this.<br />

⎛<br />

projM, N ˆ (�v )=2· ⎝ 1<br />

⎞ ⎛<br />

0⎠<br />

+(9/5) · ⎝<br />

0<br />

0<br />

⎞ ⎛<br />

2⎠<br />

= ⎝<br />

1<br />

2<br />

⎞<br />

18/5⎠<br />

9/5<br />

Therefore projection along different subspaces may yield different results.<br />

These pictures compare the two maps. Both show that the projection is<br />

indeed ‘into’ the plane and ‘along’ the line.

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