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Linear Algebra, Theory And Applications, 2012a

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134 SOME FACTORIZATIONS<br />

9. Find a QR factorization for the matrix<br />

⎛<br />

⎝ 1 2 1 0<br />

3 0 1 1<br />

1 0 2 1<br />

10. If you had a QR factorization, A = QR, describe how you could use it to solve the<br />

equation Ax = b.<br />

11. If Q is an orthogonal matrix, show the columns are an orthonormal set. That is show<br />

that for<br />

Q = ( )<br />

q 1 ··· q n<br />

it follows that q i · q j = δ ij . Also show that any orthonormal set of vectors is linearly<br />

independent.<br />

12. Show you can’t expect uniqueness for QR factorizations. Consider<br />

⎛<br />

⎝ 0 0 0 0 0<br />

1<br />

⎞<br />

⎠<br />

0 0 1<br />

and verify this equals<br />

and also<br />

⎛<br />

⎝<br />

⎞ ⎛<br />

√<br />

0 1 0<br />

2 0<br />

1<br />

√ 2√<br />

2 ⎠<br />

√<br />

2 0 −<br />

1<br />

2 2<br />

1<br />

2<br />

1<br />

2<br />

⎛<br />

⎝<br />

1 0 0<br />

0 1 0<br />

0 0 1<br />

⎞ ⎛<br />

⎠ ⎝<br />

⎞<br />

⎠<br />

⎝ 0 0 √<br />

2<br />

0 0 0<br />

0 0 0<br />

0 0 0<br />

0 0 1<br />

0 0 1<br />

Using Definition 5.7.4, can it be concluded that if A is an invertible matrix it will<br />

follow there is only one QR factorization?<br />

⎞<br />

⎠ .<br />

13. Suppose {a 1 , ··· , a n } are linearly independent vectors in R n and let<br />

A = ( a 1 ··· a n<br />

)<br />

⎞<br />

⎠<br />

Form a QR factorization for A.<br />

⎛<br />

( ) ( )<br />

a1 ··· a n = q1 ··· q n ⎜<br />

⎝<br />

⎞<br />

r 11 r 12 ··· r 1n<br />

0 r 22 ··· r 2n<br />

.<br />

. ..<br />

⎟<br />

⎠<br />

0 0 ··· r nn<br />

Show that for each k ≤ n,<br />

span (a 1 , ··· , a k ) = span (q 1 , ··· , q k )<br />

Prove that every subspace of R n has an orthonormal basis. The procedure just described<br />

is similar to the Gram Schmidt procedure which will be presented later.<br />

14. Suppose Q n R n converges to an orthogonal matrix Q where Q n is orthogonal and R n<br />

is upper triangular having all positive entries on the diagonal. Show that then Q n<br />

converges to Q and R n converges to the identity.

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