Question #254840

2. Let A=(310620310)\begin{pmatrix} - 3 & 1 & 0\\ - 6 & 2 & 0\\ - 3 & 1 & 0 \end{pmatrix}

a) Find the characteristic polynomial of A and show that the eigenvalues are 0 and - 1

b) Find a basis for each eigenspace of A

c) Explain why is A diagonalisable

d) Find an invertible matrix P and a diagonal matrix D such that A=PDP-1

e) Hence, or otherwise, calculate A2018

1
Expert's answer
2021-10-26T07:07:41-0400

a)


A=(310620310)A=\begin{pmatrix} - 3 & 1 & 0\\ - 6 & 2 & 0\\ - 3 & 1 & 0 \end{pmatrix}


AλI=(3λ1062λ0310λ)A-\lambda I=\begin{pmatrix} - 3-\lambda & 1 & 0\\ - 6 & 2-\lambda & 0\\ - 3 & 1& 0-\lambda \end{pmatrix}

Characteristic polynomial


det(AλI)=AλI=3λ1062λ0310λ\det(A-\lambda I)=|A-\lambda I|=\begin{vmatrix} - 3-\lambda & 1 & 0\\ - 6 & 2-\lambda & 0\\ - 3 & 1 & 0 -\lambda \end{vmatrix}


=(3λ)2λ01λ1603λ+062λ31=(-3-\lambda)\begin{vmatrix} 2-\lambda& 0 \\ 1 & -\lambda \end{vmatrix}-1\begin{vmatrix} -6 & 0 \\ -3& -\lambda \end{vmatrix}+0\begin{vmatrix} -6 & 2-\lambda \\ -3 & 1 \end{vmatrix}

=(3λ)(2λ+λ20)(6λ+0)+0=(-3-\lambda)(-2\lambda+\lambda^2-0)-(6\lambda+0)+0

=6λ3λ2+2λ2λ36λ=λ3λ2=6\lambda-3\lambda^2+2\lambda^2-\lambda^3-6\lambda=-\lambda^3-\lambda^2

The characteristic equation of AA is


AλI=0|A-\lambda I|=0

λ3λ2=0-\lambda^3-\lambda^2=0

The roots are λ1=λ2=0,λ3=1.\lambda_1=\lambda_2=0, \lambda_3=-1.

These are the eigenvalues.


b) Find the eigenvectors.

λ=1\lambda=-1


AλI=(210630311)A-\lambda I=\begin{pmatrix} - 2 & 1 & 0\\ - 6 & 3 & 0\\ - 3 & 1& 1 \end{pmatrix}

R2=R23R1R_2=R_2-3R_1


(210000311)\begin{pmatrix} - 2 & 1 & 0\\ 0 & 0 & 0\\ - 3 & 1& 1 \end{pmatrix}

R3=R33R1/2R_3=R_3-3R_1/2


(21000001/21)\begin{pmatrix} - 2 & 1 & 0\\ 0 & 0 & 0\\ 0 & -1/2& 1 \end{pmatrix}

Swap the rows 2 and 3


(21001/21000)\begin{pmatrix} - 2 & 1 & 0\\ 0 & -1/2 & 1\\ 0 & 0 & 0 \end{pmatrix}

R2=2R2R_2=-2R_2


(210012000)\begin{pmatrix} - 2 & 1 & 0\\ 0 & 1 & -2\\ 0 & 0 & 0 \end{pmatrix}

R1=R1R2R_1=R_1-R_2


(202012000)\begin{pmatrix} - 2 & 0 & 2\\ 0 & 1 & -2\\ 0 & 0 & 0 \end{pmatrix}

R1=R1/(2)R_1=R_1/(-2)


(101012000)\begin{pmatrix} 1 & 0 & -1\\ 0 & 1 & -2\\ 0 & 0 & 0 \end{pmatrix}

Solve the matrix equation


(101012000)(x1x2x3)=(000)\begin{pmatrix} 1 & 0 & -1\\ 0 & 1 & -2\\ 0 & 0 & 0 \end{pmatrix}\begin{pmatrix} x_1\\ x_2\\ x_3 \end{pmatrix}=\begin{pmatrix} 0\\ 0\\ 0 \end{pmatrix}

If we take x3=t,x_3=t, then x1=t,x2=2t.x_1=t, x_2=2t.

Thus

x=(t2tt)=(121)t\vec x=\begin{pmatrix} t\\ 2t\\ t \end{pmatrix}=\begin{pmatrix} 1\\ 2\\ 1 \end{pmatrix}t

The null space of this matrix is 


{(121)}\bigg\{\begin{pmatrix} 1\\ 2\\ 1 \end{pmatrix}\bigg\}



λ=0\lambda=0

AλI=(310620310)A-\lambda I=\begin{pmatrix} - 3 & 1 & 0\\ - 6 & 2 & 0\\ - 3 & 1 & 0 \end{pmatrix}

R2=R22R1R_2=R_2-2R_1

(310000310)\begin{pmatrix} - 3 & 1 & 0\\ 0 & 0 & 0\\ - 3 & 1& 0 \end{pmatrix}

R3=R3R1R_3=R_3-R_1

(310000000)\begin{pmatrix} - 3 & 1 & 0\\ 0 & 0 & 0\\ 0 & 0& 0 \end{pmatrix}


R1=R1/(3)R_1=R_1/(-3)


(11/30000000)\begin{pmatrix} 1 & -1/3 & 0\\ 0 & 0 & 0\\ 0 & 0& 0 \end{pmatrix}

Solve the matrix equation


(11/30000000)(x1x2x3)=(000)\begin{pmatrix} 1 & -1/3 & 0\\ 0 & 0 & 0\\ 0 & 0& 0 \end{pmatrix}\begin{pmatrix} x_1\\ x_2\\ x_3 \end{pmatrix}=\begin{pmatrix} 0\\ 0\\ 0 \end{pmatrix}

If we take x2=t,x3=s,x_2=t,x_3=s, then x1=(1/3)t.x_1=(1/3)t.

Thus

x=((1/3)tts)=(1/310)t+(001)s\vec x=\begin{pmatrix} (1/3)t\\ t\\ s \end{pmatrix}=\begin{pmatrix} 1/3\\ 1\\ 0 \end{pmatrix}t+\begin{pmatrix} 0\\ 0\\ 1 \end{pmatrix}s


The null space of this matrix is 


{(1/310),(001)}\bigg\{\begin{pmatrix} 1/3\\ 1\\ 0 \end{pmatrix},\begin{pmatrix} 0\\ 0\\ 1 \end{pmatrix}\bigg\}

Eigenvalue: 1,−1, multiplicity: 1,1, eigenvector: (121)\begin{pmatrix} 1\\ 2\\ 1 \end{pmatrix}

Eigenvalue: 0,0, multiplicity: 2,2, eigenvectors: (1/310),(001)\begin{pmatrix} 1/3\\ 1\\ 0 \end{pmatrix},\begin{pmatrix} 0\\ 0\\ 1 \end{pmatrix}


c) A matrix is diagonalizable if and only of for each eigenvalue the dimension of the eigenspace is equal to the multiplicity of the eigenvalue.

For the eigenvalue 1-1 this is trivially true as its multiplicity is only one and we find one nonzero eigenvector associated to it.

For the eigenvector  we find two linearly indepedent eigenvectors.

Therefore the matrix AA is diagonalizable.


d)

Form the matrix P,P, whose column ii  is ii-th eigenvector


P=(11/30210101)P=\begin{pmatrix} 1 & 1/3 & 0\\ 2 & 1 & 0\\ 1 & 0& 1 \end{pmatrix}

Form the diagonal matrix DD whose element at row i,i, column ii is ii-th eigenvalue


D=(100000000)D=\begin{pmatrix} -1 & 0& 0\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{pmatrix}

detP=11/30210101=11001(1/3)2011+0\det P=\begin{vmatrix} 1 & 1/3 & 0\\ 2 & 1 & 0\\ 1 & 0& 1 \end{vmatrix}=1\begin{vmatrix} 1 & 0 \\ 0 & 1 \end{vmatrix}-(1/3)\begin{vmatrix} 2 & 0 \\ 1 & 1 \end{vmatrix}+0

=12/3=1/30=1-2/3=1/3\not=0

The cofactor matrix is


(1211/311/3001/3)\begin{pmatrix} 1 & -2& -1\\ -1/3 & 1 & 1/3\\ 0 & 0 & 1/3 \end{pmatrix}

The adjugate matrix is


(11/3021011/31/3)\begin{pmatrix} 1 & -1/3 & 0\\ -2 & 1 & 0\\ -1 & 1/3 & 1/3 \end{pmatrix}

The inverse matrix is the adjugate matrix divided by the determinant.


P1=(310630311)P^{-1}=\begin{pmatrix} 3 & -1 & 0\\ -6 & 3 & 0\\ -3 & 1 & 1 \end{pmatrix}

A=PDP1A=PDP^{-1}


A2018=PD2018P1A^{2018}=PD^{2018}P^{-1}

=(11/30210101)((1)201800000000)(310630311)=\begin{pmatrix} 1 & 1/3 & 0\\ 2 & 1 & 0\\ 1 & 0& 1 \end{pmatrix}\begin{pmatrix} (-1)^{2018} & 0& 0\\ 0 & 0 & 0\\ 0 & 0 & 0 \end{pmatrix}\begin{pmatrix} 3 & -1 & 0\\ -6 & 3 & 0\\ -3 & 1 & 1 \end{pmatrix}


=(100200100)(310630311)=\begin{pmatrix} 1 & 0 & 0\\ 2 & 0 & 0\\ 1 & 0 & 0 \end{pmatrix}\begin{pmatrix} 3 & -1 & 0\\ -6 & 3 & 0\\ -3 & 1 & 1 \end{pmatrix}

=(310620310)=\begin{pmatrix} 3 & -1 & 0\\ 6 & -2 & 0\\ 3 & -1 & 0 \end{pmatrix}


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