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Linear Algebra, Theory And Applications, 2012a

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5.8. EXERCISES 133<br />

Theorem 5.7.5 Let A be any real m × n matrix. Then there exists an orthogonal matrix<br />

Q and an upper triangular matrix R having nonnegative entries on the main diagonal such<br />

that<br />

A = QR<br />

and this factorization can be accomplished in a systematic manner.<br />

◮◮<br />

5.8 Exercises<br />

⎛ ⎞<br />

1. Find a LU factorization of ⎝ 1 2 0<br />

2 1 3 ⎠ .<br />

1 2 3<br />

2. Find a LU factorization of<br />

⎛<br />

⎝ 1 1 2 3 3 2 2<br />

1<br />

⎞<br />

⎠ .<br />

5 0 1 3<br />

⎛ ⎞<br />

3. Find a PLU factorization of ⎝ 1 2 1<br />

1 2 2 ⎠ .<br />

2 1 1<br />

⎛<br />

1 2 1 2<br />

⎞<br />

1<br />

4. Find a PLU factorization of ⎝ 2 4 2 4 1 ⎠ .<br />

1 2 1 3 2<br />

⎛ ⎞<br />

1 2 1<br />

5. Find a PLU factorization of ⎜ 1 2 2<br />

⎟<br />

⎝ 2 4 1 ⎠ .<br />

3 2 1<br />

6. Is there only one LU factorization for a given matrix? Hint: Consider the equation<br />

( ) ( )( )<br />

0 1 1 0 0 1<br />

=<br />

.<br />

0 1 1 1 0 0<br />

7. Here is a matrix and an LU factorization of it.<br />

⎛<br />

A = ⎝ 1 2 5 0<br />

⎞ ⎛<br />

1 1 4 9 ⎠ = ⎝ 1 0 0<br />

1 1 0<br />

0 1 2 5 0 −1 1<br />

⎞ ⎛<br />

⎠<br />

Use this factorization to solve the system of equations<br />

⎛<br />

Ax = ⎝ 1 ⎞<br />

2 ⎠<br />

3<br />

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

⎛<br />

⎝ 1 2 1<br />

3 −2 1<br />

1 0 2<br />

⎞<br />

⎠<br />

⎝ 1 2 5 0<br />

0 −1 −1 9<br />

0 0 1 14<br />

⎞<br />

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