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Mathematical Methods for Physics and Engineering: A Comprehensive Guide

K. F. Riley, M. P. Hobson, S. J. Bence

Chapter 8

Matrices and vector spaces - all with Video Answers

Educators


Chapter Questions

06:59

Problem 1

Which of the following statements about linear vector spaces are true? Where a statement is false, give a counter-example to demonstrate this.
(a) Non-singular $N \times N$ matrices form a vector space of dimension $N^{2}$.
(b) Singular $N \times N$ matrices form a vector space of dimension $N^{2}$.
(c) Complex numbers form a vector space of dimension $2 .$
(d) Polynomial functions of $x$ form an infinite-dimensional vector space.
(e) Series $\left\{a_{0}, a_{1}, a_{2}, \ldots, a_{N}\right\}$ for which $\sum_{n=0}^{N}\left|a_{n}\right|^{2}=1$ form an $N$-dimensional vector space.
(f) Absolutely convergent series form an infinite-dimensional vector space.
(g) Convergent series with terms of alternating sign form an infinite-dimensional vector space.

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
09:09

Problem 2

Evaluate the determinants
(a) $\left|\begin{array}{lll}a & h & g \\ h & b & f \\ g & f & c\end{array}\right|$
(b) $\left|\begin{array}{cccc}1 & 0 & 2 & 3 \\ 0 & 1 & -2 & 1 \\ 3 & -3 & 4 & -2 \\ -2 & 1 & -2 & 1\end{array}\right|$
and
(c) $\left|\begin{array}{cccc}g c & g e & a+g e & g b+g e \\ 0 & b & b & b \\ c & e & e & b+e \\ a & b & b+f & b+d\end{array}\right|$

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
04:43

Problem 3

Using the properties of determinants, solve with a minimum of calculation the following equations for $x$ :
(a) $\left|\begin{array}{llll}x & a & a & 1 \\ a & x & b & 1 \\ a & b & x & 1 \\ a & b & c & 1\end{array}\right|=0$
(b) $\left|\begin{array}{ccc}x+2 & x+4 & x-3 \\ x+3 & x & x+5 \\ x-2 & x-1 & x+1\end{array}\right|=0$

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
13:30

Problem 4

Consider the matrices
(a) $\mathrm{B}=\left(\begin{array}{ccc}0 & -i & i \\ i & 0 & -i \\ -i & i & 0\end{array}\right)$
(b) $\mathrm{C}=\frac{1}{\sqrt{8}}\left(\begin{array}{ccc}\sqrt{3} & -\sqrt{2} & -\sqrt{3} \\ 1 & \sqrt{6} & -1 \\ 2 & 0 & 2\end{array}\right)$
Are they (i) real, (ii) diagonal, (iii) symmetric, (iv) antisymmetric, (v) singular, (vi) orthogonal, (vii) Hermitian, (viii) anti-Hermitian, (ix) unitary, $(\mathrm{x})$ normal?

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
02:24

Problem 5

By considering the matrices
$$
\mathrm{A}=\left(\begin{array}{ll}
1 & 0 \\
0 & 0
\end{array}\right), \quad \mathrm{B}=\left(\begin{array}{ll}
0 & 0 \\
3 & 4
\end{array}\right)
$$
show that $A B=0$ does not imply that either $A$ or $B$ is the zero matrix but that it does imply that at least one of them is singular.

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
View

Problem 6

(a) The basis vectors of the unit cell of a crystal, with the origin $O$ at one corner, are denoted by $\mathbf{e}_{1}, \mathbf{e}_{2}, \mathbf{e}_{3} .$ The matrix G has elements $G_{i j}$, where $G_{i j}=\mathbf{e}_{i} \cdot \mathbf{e}_{j}$ and $H_{i j}$ are the elements of the matrix $\mathrm{H} \equiv \mathrm{G}^{-1}$. Show that the vectors $\mathbf{f}_{i}=\sum_{j} H_{i j} \mathbf{e}_{j}$ are the reciprocal vectors and that $H_{i j}=\mathbf{f}_{i} \cdot \mathbf{f}_{j}$
(b) If the vectors $\mathbf{u}$ and $\mathbf{v}$ are given by
$$
\mathbf{u}=\sum_{i} u_{i} \mathbf{e}_{i}, \quad \mathbf{v}=\sum_{i} v_{i} \mathbf{f}_{i}
$$
obtain expressions for $|\mathbf{u}|,|\mathbf{v}|$, and $\mathbf{u} \cdot \mathbf{v}$
(c) If the basis vectors are each of length $a$ and the angle between each pair is $\pi / 3$, write down $\mathrm{G}$ and hence obtain $\mathrm{H}$.
(d) Calculate (i) the length of the normal from $O$ onto the plane containing the points $p^{-1} \mathbf{e}_{1}, q^{-1} \mathbf{e}_{2}, r^{-1} \mathbf{e}_{3}$, and (ii) the angle between this normal and $\mathbf{e}_{1}$.

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
06:58

Problem 7

(a) Show that if $A$ is Hermitian and $U$ is unitary then $U^{-1} \mathrm{AU}$ is Hermitian.
(b) Show that if $A$ is anti-Hermitian then $i A$ is Hermitian.
(c) Prove that the product of two Hermitian matrices $A$ and $B$ is Hermitian if and only if $A$ and $B$ commute.
(d) Prove that if $\mathrm{S}$ is a real antisymmetric matrix then $\mathrm{A}=(\mathrm{I}-\mathrm{S})(\mathrm{I}+\mathrm{S})^{-1}$ is orthogonal. If $A$ is given by
$$
A=\left(\begin{array}{cc}
\cos \theta & \sin \theta \\
-\sin \theta & \cos \theta
\end{array}\right)
$$
then find the matrix $\mathrm{S}$ that is needed to express $\mathrm{A}$ in the above form.
(e) If $\mathrm{K}$ is skew-hermitian, i.e. $\mathrm{K}^{\dagger}=-\mathrm{K}$, prove that $\mathrm{V}=(\mathrm{I}+\mathrm{K})(\mathrm{I}-\mathrm{K})^{-1}$ is unitary.

Noor Aldeen Almusleh
Noor Aldeen Almusleh
Numerade Educator
02:23

Problem 8

A and $B$ are real non-zero $3 \times 3$ matrices and satisfy the equation
$$
(A B)^{T}+B^{-1} A=0
$$
(a) Prove that if $B$ is orthogonal then $A$ is antisymmetric.
(b) Without assuming that $B$ is orthogonal, prove that $A$ is singular.

Urvashi Arora
Urvashi Arora
Numerade Educator
02:56

Problem 9

The commutator [X, Y] of two matrices is defined by the equation
$$
[X, Y]=X Y-Y X
$$
Two anti-commuting matrices $A$ and $B$ satisfy
$$
\mathrm{A}^{2}=\mathrm{I}, \quad \mathrm{B}^{2}=\mathrm{I}, \quad[\mathrm{A}, \mathrm{B}]=2 i \mathrm{C}
$$
(a) Prove that $\mathrm{C}^{2}=\mathrm{I}$ and that $[\mathrm{B}, \mathrm{C}]=2 i \mathrm{~A}$.
(b) Evaluate $[[[A, B],[B, C]],[A, B]]$.

Geena Pullo
Geena Pullo
Numerade Educator
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Problem 10

The four matrices $\mathrm{S}_{x}, \mathrm{~S}_{y}, \mathrm{~S}_{z}$ and $\mathrm{I}$ are defined by
$$
\begin{array}{ll}
\mathrm{S}_{x}=\left(\begin{array}{cc}
0 & 1 \\
1 & 0
\end{array}\right), & \mathrm{S}_{y}=\left(\begin{array}{cc}
0 & -i \\
i & 0
\end{array}\right) \\
\mathrm{S}_{z} & =\left(\begin{array}{cc}
1 & 0 \\
0 & -1
\end{array}\right), & \mathrm{I}=\left(\begin{array}{cc}
1 & 0 \\
0 & 1
\end{array}\right)
\end{array}
$$
where $i^{2}=-1$. Show that $\mathrm{S}_{x}^{2}=\mathrm{I}$ and $\mathrm{S}_{x} \mathrm{~S}_{y}=i \mathrm{~S}_{z}$, and obtain similar results by permutting $x, y$ and $z$. Given that $\mathbf{v}$ is a vector with Cartesian components $\left(v_{x}, v_{y}, v_{z}\right)$, the matrix $\mathrm{S}(\mathbf{v})$ is defined as
$$
\mathrm{S}(\mathbf{v})=v_{x} \mathrm{~S}_{x}+v_{y} \mathrm{~S}_{y}+v_{z} \mathrm{~S}_{z}
$$
Prove that, for general non-zero vectors a and b,
$$
\mathrm{S}(\mathbf{a}) \mathrm{S}(\mathbf{b})=\mathbf{a} \cdot \mathbf{b} \mid+i \mathrm{~S}(\mathbf{a} \times \mathbf{b})
$$
Without further calculation, deduce that $\mathrm{S}(\mathbf{a})$ and $\mathrm{S}(\mathbf{b})$ commute if and only if a and $\mathbf{b}$ are parallel vectors.

Anecia Mcmurrin-Bala
Anecia Mcmurrin-Bala
Numerade Educator
00:29

Problem 11

A general triangle has angles $\alpha, \beta$ and $\gamma$ and corresponding opposite sides $a$, $b$ and $c .$ Express the length of each side in terms of the lengths of the other two sides and the relevant cosines, writing the relationships in matrix and vector form using the vectors having components $a, b, c$ and $\cos \alpha, \cos \beta, \cos \gamma .$ Invert the matrix and hence deduce the cosine-law expressions involving $\alpha, \beta$ and $\gamma$.

Allison Knapp
Allison Knapp
Numerade Educator
07:34

Problem 12

Given a matrix
$$
\mathrm{A}=\left(\begin{array}{lll}
1 & \alpha & 0 \\
\beta & 1 & 0 \\
0 & 0 & 1
\end{array}\right)
$$
where $\alpha$ and $\beta$ are non-zero complex numbers, find its eigenvalues and eigenvectors. Find the respective conditions for (a) the eigenvalues to be real and (b) the eigenvectors to be orthogonal. Show that the conditions are jointly satisfied if and only if $A$ is Hermitian.

Sam Stansfield
Sam Stansfield
Numerade Educator
01:42

Problem 13

Using the Gram-Schmidt procedure:
(a) construct an orthonormal set of vectors from the following:
(b) find an orthonormal basis, within a four-dimensional Euclidean space, for the subspace spanned by the three vectors $\left(\begin{array}{llll}1 & 2 & 0 & 0\end{array}\right)^{\mathrm{T}},(3 \quad-1 \quad 2 \quad 0)^{\mathrm{T}}$ and $\left.\begin{array}{llll}0 & 0 & 2 & 1\end{array}\right)^{\mathrm{T}}$.

Wendi Zhao
Wendi Zhao
Numerade Educator
04:01

Problem 14

If a unitary matrix $\mathrm{U}$ is written as $\mathrm{A}+i \mathrm{~B}$, where $\mathrm{A}$ and $\mathrm{B}$ are Hermitian with non-degenerate eigenvalues, show the following:
(a) A and B commute;
(b) $A^{2}+B^{2}=1$
(c) The eigenvectors of $A$ are also eigenvectors of $B$;
(d) The eigenvalues of $U$ have unit modulus (as is necessary for any unitary matrix).

WM
William Mead
Numerade Educator
02:46

Problem 15

Determine which of the matrices below are mutually commuting, and, for those that are, demonstrate that they have a complete set of eigenfunctions in common:
$$
\begin{array}{ll}
\mathrm{A}=\left(\begin{array}{cc}
6 & -2 \\
-2 & 9
\end{array}\right), & \mathrm{B}=\left(\begin{array}{cc}
1 & 8 \\
8 & -11
\end{array}\right) \\
\mathrm{C}=\left(\begin{array}{cc}
-9 & -10 \\
-10 & 5
\end{array}\right), & \mathrm{D}=\left(\begin{array}{cc}
14 & 2 \\
2 & 11
\end{array}\right)
\end{array}
$$

James Kiss
James Kiss
Numerade Educator
02:50

Problem 16

Find the eigenvalues and a set of eigenvectors of the matrix
$$
\left(\begin{array}{ccc}
1 & 3 & -1 \\
3 & 4 & -2 \\
-1 & -2 & 2
\end{array}\right)
$$
Verify that its eigenvectors are mutually orthogonal.

Minh Le
Minh Le
Numerade Educator
05:49

Problem 17

Find three real orthogonal column matrices, each of which is a simultaneous eigenvector of
$$
\mathrm{A}=\left(\begin{array}{lll}
0 & 0 & 1 \\
0 & 1 & 0 \\
1 & 0 & 0
\end{array}\right) \quad \text { and } \quad \mathrm{B}=\left(\begin{array}{lll}
0 & 1 & 1 \\
1 & 0 & 1 \\
1 & 1 & 0
\end{array}\right)
$$

Anthony Ramos
Anthony Ramos
Numerade Educator
04:58

Problem 18

Use the results of the first worked example in section $8.14$ to evaluate, without repeated matrix multiplication, the expression $A^{6} x$, where $x=\left(\begin{array}{lll}2 & 4 & -1\end{array}\right)^{\mathrm{T}}$ and A is the matrix given in the example.

James Kiss
James Kiss
Numerade Educator
04:36

Problem 19

Given that $A$ is a real symmetric matrix with normalised eigenvectors $\mathrm{e}^{i}$ obtain the coefficients $\alpha_{i}$ involved when column matrix $x$, which is the solution of
$$
\mathrm{A} \mathrm{x}-\mu \mathrm{x}=\mathrm{v}
$$
is expanded as $x=\sum_{i} \alpha_{i} e^{i} .$ Here $\mu$ is a given constant and $v$ is a given column matrix.
(a) Solve (*) when
$$
\mathrm{A}=\left(\begin{array}{lll}
2 & 1 & 0 \\
1 & 2 & 0 \\
0 & 0 & 3
\end{array}\right)
$$
$\mu=2$ and $\mathrm{v}=\left(\begin{array}{lll}1 & 2 & 3\end{array}\right)^{\mathrm{T}}$
(b) Would $(*)$ have a solution if $\mu=1$ and (i) $v=\left(\begin{array}{lll}1 & 2 & 3\end{array}\right)^{\mathrm{T}}$, (ii) $\mathrm{v}=$ $\left(\begin{array}{lll}2 & 2 & 3\end{array}\right)^{\mathrm{T}} ?$

Sam Stansfield
Sam Stansfield
Numerade Educator
03:32

Problem 20

Demonstrate that the matrix
$$
\mathrm{A}=\left(\begin{array}{ccc}
2 & 0 & 0 \\
-6 & 4 & 4 \\
3 & -1 & 0
\end{array}\right)
$$
is defective, i.e. does not have three linearly independent eigenvectors, by showing the following:
(a) its eigenvalues are degenerate and, in fact, all equal;
(b) any eigenvector has the form $\left(\begin{array}{lll}\mu & (3 \mu-2 v) & v\end{array}\right)^{\mathrm{T}}$.
(c) if two pairs of values, $\mu_{1}, v_{1}$ and $\mu_{2}, v_{2}$, define two independent eigenvectors $\mathrm{v}_{1}$ and $\mathrm{v}_{2}$ then any third similarly defined eigenvector $\mathrm{v}_{3}$ can be written as a linear combination of $\mathrm{v}_{1}$ and $\mathrm{v}_{2}$, i.e.
$$
\mathbf{v}_{3}=a \mathbf{v}_{1}+b \mathbf{v}_{2}
$$
where
$$
a=\frac{\mu_{3} v_{2}-\mu_{2} v_{3}}{\mu_{1} v_{2}-\mu_{2} v_{1}} \quad \text { and } \quad b=\frac{\mu_{1} v_{3}-\mu_{3} v_{1}}{\mu_{1} v_{2}-\mu_{2} v_{1}}
$$
Illustrate (c) using the example $\left(\mu_{1}, v_{1}\right)=(1,1),\left(\mu_{2}, v_{2}\right)=(1,2)$ and $\left(\mu_{3}, v_{3}\right)=$ $(0,1)$
Show further that any matrix of the form
$$
\left(\begin{array}{ccc}
2 & 0 & 0 \\
6 n-6 & 4-2 n & 4-4 n \\
3-3 n & n-1 & 2 n
\end{array}\right)
$$
is defective, with the same eigenvalues and eigenvectors as $A$.

Jimmy Yao
Jimmy Yao
Numerade Educator
07:51

Problem 21

By finding the eigenvectors of the Hermitian matrix
$$
\mathrm{H}=\left(\begin{array}{cc}
10 & 3 i \\
-3 i & 2
\end{array}\right)
$$
construct a unitary matrix $\mathrm{U}$ such that $\mathrm{U}^{\dagger} \mathrm{HU}=\Lambda$, where $\Lambda$ is a real diagonal matrix.

Sam Stansfield
Sam Stansfield
Numerade Educator
02:35

Problem 22

Use the stationary properties of quadratic forms to determine the maximum and minimum values taken by the expression
$$
Q=5 x^{2}+4 y^{2}+4 z^{2}+2 x z+2 x y
$$
on the unit sphere $x^{2}+y^{2}+z^{2}=1$. For what values of $x, y, z$ do they occur?

Lucas Finney
Lucas Finney
Numerade Educator
04:36

Problem 23

Given that the matrix
$$
\mathrm{A}=\left(\begin{array}{ccc}
2 & -1 & 0 \\
-1 & 2 & -1 \\
0 & -1 & 2
\end{array}\right)
$$
has two eigenvectors of the form $(1 \quad y \quad 1)^{\mathrm{T}}$, use the stationary property of the expression $J(\mathrm{x})=\mathrm{x}^{\mathrm{T}} \mathrm{Ax} /\left(\mathrm{x}^{\mathrm{T}} \mathrm{x}\right)$ to obtain the corresponding eigenvalues. Deduce the third eigenvalue.

Sam Stansfield
Sam Stansfield
Numerade Educator
07:17

Problem 24

Find the lengths of the semi-axes of the ellipse
$$
73 x^{2}+72 x y+52 y^{2}=100
$$
and determine its orientation.

Zach Steedman
Zach Steedman
Numerade Educator
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Problem 25

The equation of a particular conic section is
$$
Q \equiv 8 x_{1}^{2}+8 x_{2}^{2}-6 x_{1} x_{2}=110
$$
Determine the type of conic section this represents, the orientation of its principal axes, and relevant lengths in the directions of these axes.

Victor Salazar
Victor Salazar
Numerade Educator
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Problem 26

Show that the quadratic surface
$$
5 x^{2}+11 y^{2}+5 z^{2}-10 y z+2 x z-10 x y=4
$$
is an ellipsoid with semi-axes of lengths 2,1 and $0.5$. Find the direction of its longest axis.

Victor Salazar
Victor Salazar
Numerade Educator
01:02

Problem 27

Find the direction of the axis of symmetry of the quadratic surface
$$
7 x^{2}+7 y^{2}+7 z^{2}-20 y z-20 x z+20 x y=3
$$

Raj Bala
Raj Bala
Numerade Educator
01:00

Problem 28

Find the eigenvalues, and sufficient of the eigenvectors, of the following matrices to be able to describe the quadratic surfaces associated with them.
(a) $\left(\begin{array}{ccc}5 & 1 & -1 \\ 1 & 5 & 1 \\ -1 & 1 & 5\end{array}\right)$
(b) $\left(\begin{array}{lll}1 & 2 & 2 \\ 2 & 1 & 2 \\ 2 & 2 & 1\end{array}\right)$.
(c) $\left(\begin{array}{ccc}1 & 2 & 1 \\ 2 & 4 & 2 \\ -1 & 2 & 1\end{array}\right)$.

Raj Bala
Raj Bala
Numerade Educator
09:34

Problem 29

(a) Rearrange the result $\mathrm{A}^{\prime}=\mathrm{S}^{-1} \mathrm{AS}$ of section $8.16$ to express the original matrix $A$ in terms of the unitary matrix $S$ and the diagonal matrix $A^{\prime}$. Hence show how to construct a matrix $A$ that has given eigenvalues and given (orthogonal) column matrices as its eigenvectors.
(b) Find the matrix with eigenvectors $\left(\begin{array}{lllll}1 & 2 & 1\end{array}\right)^{\mathrm{T}},\left(\begin{array}{llll}1 & -1 & 1\end{array}\right)^{\mathrm{T}}$ and $\left(\begin{array}{ccc}1 & 0 & -1\end{array}\right)^{\mathrm{T}}$, and corresponding eigenvalues $\lambda, \mu$ and $v$.
(c) Try a particular case, say $\lambda=3, \mu=-2$ and $v=1$, and verify by explicit solution that the matrix so found does have these eigenvalues.

Matthew Allcock
Matthew Allcock
Numerade Educator
02:37

Problem 30

Find an orthogonal transformation that takes the quadratic form
$$
Q \equiv-x_{1}^{2}-2 x_{2}^{2}-x_{3}^{2}+8 x_{2} x_{3}+6 x_{1} x_{3}+8 x_{1} x_{2}
$$
into the form
$$
\mu_{1} y_{1}^{2}+\mu_{2} y_{2}^{2}-4 y_{3}^{2}
$$
and determine $\mu_{1}$ and $\mu_{2}$ (see section 8.17).

Anand Jangid
Anand Jangid
Numerade Educator
02:46

Problem 31

One method of determining the nullity (and hence the rank) of an $M \times N$ matrix A is as follows.
- Write down an augmented transpose of $A$, by adding on the right an $N \times N$ unit matrix and thus producing an $N \times(M+N)$ array $\mathrm{B}$.
- Subtract a suitable multiple of the first row of $B$ from each of the other lower rows so as to make $B_{i 1}=0$ for $i>1$
- Subtract a suitable multiple of the second row (or the uppermost row that does not start with $M$ zero values) from each of the other lower rows so as to make $B_{i 2}=0$ for $i>2$
- Continue in this way until all remaining rows have zeroes in the first $M$ places. The number of such rows is equal to the nullity of $A$ and the $N$ rightmost entries of these rows are the components of vectors that span the null space. They can be made orthogonal if they are not so already.
Use this method to show that the nullity of
$$
A=\left(\begin{array}{cccc}
-1 & 3 & 2 & 7 \\
3 & 10 & -6 & 17 \\
-1 & -2 & 2 & -3 \\
2 & 3 & -4 & 4 \\
4 & 0 & -8 & -4
\end{array}\right)
$$
is 2 and that an orthogonal base for the null space of $A$ is provided by any two column matrices of the form $\left(\begin{array}{lll}2+\alpha_{i} & -2 \alpha_{i} & 1 & \alpha_{i}\end{array}\right)^{\mathrm{T}}$ for which the $\alpha_{i}(i=1,2)$ are real and satisfy $6 \alpha_{1} \alpha_{2}+2\left(\alpha_{1}+\alpha_{2}\right)+5=0$.

Varsha Aggarwal
Varsha Aggarwal
Numerade Educator
05:56

Problem 32

Do the following sets of equations have non-zero solutions? If so, find them.
(a) $3 x+2 y+z=0, \quad x-3 y+2 z=0, \quad 2 x+y+3 z=0$
(b) $2 x=b(y+z), \quad x=2 a(y-z), \quad x=(6 a-b) y-(6 a+b) z$.

Sam Stansfield
Sam Stansfield
Numerade Educator
03:55

Problem 33

Solve the simultaneous equations
$$
\begin{aligned}
2 x+3 y+z &=11 \\
x+y+z &=6 \\
5 x-y+10 z &=34
\end{aligned}
$$

Mahmoud Hammouri
Mahmoud Hammouri
Numerade Educator
05:57

Problem 34

Solve the following simultaneous equations for $x_{1}, x_{2}$ and $x_{3}$, using matrix methods:
$$
\begin{aligned}
x_{1}+2 x_{2}+3 x_{3} &=1 \\
3 x_{1}+4 x_{2}+5 x_{3} &=2 \\
x_{1}+3 x_{2}+4 x_{3} &=3
\end{aligned}
$$

Mahmoud Hammouri
Mahmoud Hammouri
Numerade Educator
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Problem 35

Show that the following equations have solutions only if $\eta=1$ or 2 , and find them in these cases:
$$
\begin{aligned}
x+y+z &=1 \\
x+2 y+4 z &=\eta \\
x+4 y+10 z &=\eta^{2}
\end{aligned}
$$

Eduard Sanchez
Eduard Sanchez
Numerade Educator
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Problem 36

Find the condition(s) on $\alpha$ such that the simultaneous equations
$$
\begin{aligned}
x_{1}+\alpha x_{2} &=1 \\
x_{1}-x_{2}+3 x_{3} &=-1 \\
2 x_{1}-2 x_{2}+\alpha x_{3} &=-2
\end{aligned}
$$
have (a) exactly one solution, (b) no solutions, or (c) an infinite number of solutions; give all solutions where they exist.

Eduard Sanchez
Eduard Sanchez
Numerade Educator
03:30

Problem 37

Make an $L U$ decomposition of the matrix
$$
A=\left(\begin{array}{ccc}
3 & 6 & 9 \\
1 & 0 & 5 \\
2 & -2 & 16
\end{array}\right)
$$
and hence solve $A x=b$, where (i) $b=\left(\begin{array}{lll}21 & 9 & 28\end{array}\right)^{\mathrm{T}}$, (ii) $b=\left(\begin{array}{lll}21 & 7 & 22\end{array}\right)^{\mathrm{T}}$.

Jacquelyn Trost
Jacquelyn Trost
Numerade Educator
03:00

Problem 38

Make an $L U$ decomposition of the matrix
$$
A=\left(\begin{array}{cccc}
2 & -3 & 1 & 3 \\
1 & 4 & -3 & -3 \\
5 & 3 & -1 & -1 \\
3 & -6 & -3 & 1
\end{array}\right)
$$
Hence solve $A x=b$ for (i) $b=\left(\begin{array}{llll}-4 & 1 & 8 & -5\end{array}\right)^{\mathrm{T}}$, (ii) $b=\left(\begin{array}{llll}-10 & 0 & -3 & -24\end{array}\right)^{\mathrm{T}}$.
Deduce that det $A=-160$ and confirm this by direct calculation.

Tamara Worner
Tamara Worner
Numerade Educator
12:55

Problem 39

Use the Cholesky separation method to determine whether the following matrices are positive definite. For each that is, determine the corresponding lower diagonal matrix L:
$$
\mathrm{A}=\left(\begin{array}{ccc}
2 & 1 & 3 \\
1 & 3 & -1 \\
3 & -1 & 1
\end{array}\right), \quad \mathrm{B}=\left(\begin{array}{ccc}
5 & 0 & \sqrt{3} \\
0 & 3 & 0 \\
\sqrt{3} & 0 & 3
\end{array}\right)
$$

Tim Strang
Tim Strang
Numerade Educator
03:41

Problem 40

Find the equation satisfied by the squares of the singular values of the matrix associated with the following over-determined set of equations:
$$
\begin{aligned}
2 x+3 y+z &=0 \\
x-y-z &=1 \\
2 x+y &=0 \\
2 y+z &=-2
\end{aligned}
$$
Show that one of the singular values is close to zero. Determine the two larger singular values by an appropriate iteration process and the smallest by indirect calculation.

Vikash Ranjan
Vikash Ranjan
Numerade Educator
04:40

Problem 41

Find the SVD of
$$
\left(\begin{array}{cc}
0 & -1 \\
1 & 1 \\
-1 & 0
\end{array}\right)
$$
showing that the singular values are $\sqrt{3}$ and 1 .

Jack Chen
Jack Chen
Numerade Educator
04:29

Problem 42

Find the SVD form of the matrix
$$
\mathrm{A}=\left(\begin{array}{ccc}
22 & 28 & -22 \\
1 & -2 & -19 \\
19 & -2 & -1 \\
-6 & 12 & 6
\end{array}\right)
$$
Hence find the best solution $x$ to the equation $A x=b$ when (i) $b=(6-$ $\left.\begin{array}{llll}39 & 15 & 18\end{array}\right)^{\mathrm{T}}$, (ii) $\mathrm{b}=\left(\begin{array}{lll}9 & -42 & 15 & 15\end{array}\right)^{\mathrm{T}}$, showing that (i) has an exact solution, but that the best solution to (ii) has a residual of $\sqrt{18}$.

Jack Chen
Jack Chen
Numerade Educator
01:03

Problem 43

Four experimental measurements of particular combinations of three physical variables, $x, y$ and $z$, gave the following inconsistent results:
$$
\begin{aligned}
13 x+22 y-13 z &=4 \\
10 x-8 y-10 z &=44 \\
10 x-8 y-10 z &=47 \\
9 x-18 y-9 z &=72
\end{aligned}
$$
Find the SVD best values for $x, y$ and $z$. Identify the null space of $A$ and hence obtain the general SVD solution.

Victor Salazar
Victor Salazar
Numerade Educator