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Section IV. Matrix Operations 223<br />

3.5 Example<br />

⎛<br />

1<br />

⎝0 0<br />

0<br />

⎞ ⎛<br />

0 1<br />

2⎠⎝4<br />

2<br />

5<br />

⎞ ⎛<br />

3 1<br />

6⎠<br />

=( ⎝0 0<br />

0<br />

⎞ ⎛<br />

0 0<br />

0⎠<br />

+ ⎝0 0<br />

0<br />

⎞ ⎛<br />

0 1<br />

2⎠)<br />

⎝4 2<br />

5<br />

⎞<br />

3<br />

6⎠<br />

0 0 0 7 8 9 0<br />

⎛<br />

1<br />

= ⎝0 0<br />

2<br />

0<br />

0 0<br />

⎞ ⎛<br />

3 0<br />

0⎠<br />

+ ⎝14 0 0 7<br />

⎞<br />

0 0<br />

16 18⎠<br />

8 9<br />

0<br />

⎛<br />

1<br />

= ⎝14 0 0<br />

⎞<br />

2 3<br />

16 18⎠<br />

0 0 0<br />

0 0 0<br />

But if the left-multiplier’s nonzero entries are in the same row then that row of<br />

the result is a combination.<br />

3.6 Example<br />

⎛<br />

1<br />

⎝0<br />

0<br />

0<br />

⎞ ⎛<br />

2 1<br />

0⎠⎝4<br />

2<br />

5<br />

⎞ ⎛<br />

3 1<br />

6⎠<br />

=( ⎝0<br />

0<br />

0<br />

⎞ ⎛<br />

0 0<br />

0⎠<br />

+ ⎝0<br />

0<br />

0<br />

⎞ ⎛<br />

2 1<br />

0⎠)<br />

⎝4<br />

2<br />

5<br />

⎞<br />

3<br />

6⎠<br />

0 0 0 7 8 9 0<br />

⎛<br />

1<br />

= ⎝0<br />

0<br />

2<br />

0<br />

0 0<br />

⎞ ⎛<br />

3 14<br />

0⎠<br />

+ ⎝ 0<br />

0 0 7<br />

⎞<br />

16 18<br />

0 0⎠<br />

8 9<br />

0<br />

⎛<br />

15<br />

0 0<br />

⎞<br />

18 21<br />

0 0 0<br />

= ⎝ 0 0 0⎠<br />

0 0 0<br />

Right-multiplication acts in the same way, with columns.<br />

These observations about matrices that are mostly zeroes extend to arbitrary<br />

matrices.<br />

3.7 Lemma In a product of two matrices G and H, the columns of GH are<br />

formed by taking G times the columns of H<br />

⎛<br />

⎞<br />

.<br />

⎜<br />

.<br />

.<br />

⎟<br />

G · ⎜�<br />

⎝h1<br />

··· �hn ⎟<br />

⎠<br />

.<br />

.<br />

.<br />

.<br />

=<br />

⎛<br />

.<br />

⎜<br />

.<br />

.<br />

⎜<br />

⎝G<br />

· �h1 ··· G · � ⎞<br />

⎟<br />

hn<br />

⎟<br />

⎠<br />

.<br />

.<br />

.<br />

.<br />

and the rows of GH are formed by taking the rows of G times H<br />

⎛ ⎞ ⎛<br />

⎞<br />

··· �g1 ··· ··· �g1 · H ···<br />

⎜ ⎟ ⎜<br />

⎟<br />

⎜ .<br />

⎝ .<br />

⎟<br />

⎠ · H = ⎜ .<br />

⎝ .<br />

⎟<br />

⎠<br />

··· �gr ···<br />

(ignoring the extra parentheses).<br />

··· �gr · H ···

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