Question #214228

Find the characteristic equation of the matrix A =

2 1 1

0 1 0

1 1 2

 and hence find the matrix

represented by A8 − 5A7 + 7A6 − 3A5 + A4 − 5A3 + 8A2 − 2A + I.


1
Expert's answer
2021-07-07T09:01:45-0400

Characteristic equation det(AλI)=0\det(A-\lambda I)=0


2λ1101λ0112λ=0\begin{vmatrix} 2-\lambda & 1 & 1 \\ 0 & 1-\lambda & 0 \\ 1 & 1 & 2-\lambda \\ \end{vmatrix}=0


(2λ)1λ012λ0012λ+01λ11=0(2-\lambda)\begin{vmatrix} 1-\lambda & 0 \\ 1 & 2-\lambda \end{vmatrix}-\begin{vmatrix} 0 & 0 \\ 1 & 2-\lambda \end{vmatrix}+\begin{vmatrix} 0 & 1-\lambda \\ 1 & 1 \end{vmatrix}=0

(2λ)2(1λ)(1λ)=0(2-\lambda)^2(1-\lambda)-(1-\lambda)=0

(1λ)(44λ+λ21)=0(1-\lambda)(4-4\lambda+\lambda^2-1)=0

(1λ)2(3λ)=0(1-\lambda)^2(3-\lambda)=0

λ1=1,λ2=1,λ3=3\lambda_1=1, \lambda_2=1, \lambda_3=3

The equation can be written as


λ35λ2+7λ3=0\lambda^3-5\lambda^2+7\lambda-3=0

According to Cayley Hamilton theorem, every matrix is the root of it's eigen matrix. Then


A35A2+7A3=0A^3-5A^2+7A-3=0

Given sum

This sum can be written as,


A85A7+7A63A5A^8 − 5A^7 + 7A^6 − 3A^5




+A45A3+8A22A+I+ A^4 − 5A^3 + 8A^2 − 2A + I

=(A35A2+7A3)(A5+A)+(A2+A+I)= (A^3 - 5A^2+7A -3)(A^5+A) + (A^2+A+I)

Since A35A2+7A3=0,A^3-5A^2+7A-3=0, then


A85A7+7A63A5A^8 − 5A^7 + 7A^6 − 3A^5




+A45A3+8A22A+I+ A^4 − 5A^3 + 8A^2 − 2A + I

=A2+A+I=A^2+A+I

A2=[211010112][211010112]A^2=\begin{bmatrix} 2 & 1 & 1 \\ 0 & 1 & 0 \\ 1 & 1 & 2 \\ \end{bmatrix}\begin{bmatrix} 2 & 1 & 1 \\ 0 & 1 & 0 \\ 1 & 1 & 2 \\ \end{bmatrix}

=[4+0+12+1+12+0+20+0+00+1+00+0+02+0+21+1+21+0+4]=\begin{bmatrix} 4+0+1 & 2+1+1 & 2+0+2 \\ 0+0+0 & 0+1+0 & 0+0+0 \\ 2+0+2 & 1+1+2 & 1+0+4 \\ \end{bmatrix}

=[544010445]=\begin{bmatrix} 5 & 4 & 4 \\ 0 & 1 & 0 \\ 4 & 4 & 5 \\ \end{bmatrix}


A85A7+7A63A5A^8 − 5A^7 + 7A^6 − 3A^5




+A45A3+8A22A+I+ A^4 − 5A^3 + 8A^2 − 2A + I

=A2+A+I=A^2+A+I

=[544010445]+[211010112]+[100010001]=\begin{bmatrix} 5 & 4 & 4 \\ 0 & 1 & 0 \\ 4 & 4 & 5 \\ \end{bmatrix}+\begin{bmatrix} 2 & 1 & 1 \\ 0 & 1 & 0 \\ 1 & 1 & 2 \\ \end{bmatrix}+\begin{bmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \\ \end{bmatrix}


=[855030558]=\begin{bmatrix} 8 & 5 & 5 \\ 0 & 3 & 0 \\ 5 & 5 & 8 \\ \end{bmatrix}


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