1.7.6 Let A be an n x n symmetric positive semidefinite matrix. Prove that A is positive definite if and only if det A # 0.

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Chapter7: Eigenvalues And Eigenvectors
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Can you help with question 1.7.6?

心
91%
1.5.1(f) Is the matrix
1 -1 0
-1 2 -1
0 -1
diagonalizable? If so, what are the corresponding diagonal matrix D and orthogonal matrix P?
In this case, show that P is indeed orthogonal.
1.6.4 Let A and P be n xn matrices with P invertible. It was shown in class that A and
P-'AP have the same eigenvalues. Use this fact to prove that if A and B are both invertible
n x n matrices, then AB and BA have the same eigenvalues.
1.7.5 Use Theorem 1.7.1 or Theorem 1.7.2 (you can find them in the lecture notes) to determine
the definiteness of the following quadratic forms:
(a) Q = af +8a3,
(b) Q= 5xỉ + 2.x1x3 + 2x +2x203 + 4x3,
(d) Q = -3x² + 2x1x2 – x3 + 4x2X3 – 8x3.
- Ix)- = O ()
1.7.6 Let A be an n x n symmetric positive semidefinite matrix. Prove that A is positive
definite if and only if det A # 0.
1.7.7(a) For what values of parameter c, is the quadratic form
Q(x, y) = 3x2 – (5+ c)xy + 2cy²
(i) positive definite, (ii) positive semidefinite, and (iii) indefinite?
3:16 PM
2/22/2021
Transcribed Image Text:心 91% 1.5.1(f) Is the matrix 1 -1 0 -1 2 -1 0 -1 diagonalizable? If so, what are the corresponding diagonal matrix D and orthogonal matrix P? In this case, show that P is indeed orthogonal. 1.6.4 Let A and P be n xn matrices with P invertible. It was shown in class that A and P-'AP have the same eigenvalues. Use this fact to prove that if A and B are both invertible n x n matrices, then AB and BA have the same eigenvalues. 1.7.5 Use Theorem 1.7.1 or Theorem 1.7.2 (you can find them in the lecture notes) to determine the definiteness of the following quadratic forms: (a) Q = af +8a3, (b) Q= 5xỉ + 2.x1x3 + 2x +2x203 + 4x3, (d) Q = -3x² + 2x1x2 – x3 + 4x2X3 – 8x3. - Ix)- = O () 1.7.6 Let A be an n x n symmetric positive semidefinite matrix. Prove that A is positive definite if and only if det A # 0. 1.7.7(a) For what values of parameter c, is the quadratic form Q(x, y) = 3x2 – (5+ c)xy + 2cy² (i) positive definite, (ii) positive semidefinite, and (iii) indefinite? 3:16 PM 2/22/2021
Expert Solution
Step 1

Definiteness of matrices:

The n×n matrix A is said to be positive semi definite matrix if it satisfies the following conditions,

xTAx0 for every xn

If the inequality is strict, such matrix is called as positive definite.

 

It is given A is a n×n symmetric positive definite matrix.

Every symmetric matrix has only real eigen values. Hence, the matrix A has only real eigen values.

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