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Linear Algebra, Theory And Applications, 2012a

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15.3. EXERCISES 401<br />

where A k = Q k R k . Therefore as shown before,<br />

A k+1 = R k A k R −1<br />

k<br />

Let the ij th entry of A k be a k ij .Thenifi − j ≥ 2<br />

a k+1<br />

ij =<br />

n∑<br />

j∑<br />

p=i q=1<br />

r ip a k pqr −1<br />

qj<br />

It is given that a k pq = 0 whenever p − q ≥ 2. However, from the above sum,<br />

p − q ≥ i − j ≥ 2<br />

and so the sum equals 0.<br />

Since upper Hessenberg matrices stay that way in the algorithm and it is closer to<br />

being upper triangular, it is reasonable to suppose the QR algorithm will yield good results<br />

more quickly for this upper Hessenberg matrix than for the original matrix. This would be<br />

especially true if the matrix is good sized. The other important thing to observe is that,<br />

starting with an upper Hessenberg matrix, the algorithm will restrict the size of the blocks<br />

which occur to being 2 × 2 blocks which are easy to deal with. These blocks allow you to<br />

identify the complex roots.<br />

15.3 Exercises<br />

In these exercises which call for a computation, don’t waste time on them unless you use a<br />

computer or calculator which can raise matrices to powers and take QR factorizations.<br />

1. In Example 15.1.10 an eigenvalue was found correct to several decimal places along<br />

with an eigenvector. Find the other eigenvalues along with their eigenvectors.<br />

2. Find the eigenvalues and eigenvectors of the matrix A =<br />

⎛<br />

⎝ 3 2 2 1 1<br />

3<br />

⎞<br />

⎠ numerically.<br />

1 3 2<br />

In this case the exact eigenvalues are ± √ 3, 6. Compare with the exact answers.<br />

⎛<br />

3 2 1<br />

⎞<br />

3. Find the eigenvalues and eigenvectors of the matrix A = ⎝ 2 5 3 ⎠ numerically.<br />

1 3 2<br />

The exact eigenvalues are 2, 4+ √ 15, 4 − √ 15. Compare your numerical results with<br />

the exact values. Is it much fun to compute the exact eigenvectors?<br />

⎛<br />

0 2 1<br />

⎞<br />

4. Find the eigenvalues and eigenvectors of the matrix A = ⎝ 2 5 3 ⎠ numerically.<br />

1 3 2<br />

I don’t know the exact eigenvalues in this case. Check your answers by multiplying<br />

your numerically computed eigenvectors by the matrix.<br />

5. Find the eigenvalues and eigenvectors of the matrix A =<br />

⎛<br />

⎝ 0 2 2 0 1<br />

3<br />

⎞<br />

⎠ numerically.<br />

1 3 2<br />

I don’t know the exact eigenvalues in this case. Check your answers by multiplying<br />

your numerically computed eigenvectors by the matrix.

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