Question

In: Advanced Math

Jordan Canonical Form

Let A be a square matrix defined by \( A = \begin{pmatrix}3&2\\ 3&-2\end{pmatrix} \)

(a) Find the characteristic polynomial of A.

(b) Show that A is diagonalizable then diagonalize it.

(c) Write \( A^n \) in term of n.

Solutions

Expert Solution

Solution

(a) Find the characteristic polynomial of A.

\( \implies P(\lambda)=|A-\lambda I|=\lambda^2+\lambda-12=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)

Therefore, the characteristics polynomial is 

\( P(\lambda)=|A-\lambda I|=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)

(b) Show that A is diagonalizable then diagonalize it.

since, \( P(\lambda)\hspace{2mm}splitted \implies A \) is diagonalizable.

\( \bullet\hspace{2mm} For \hspace{2mm}\lambda_1=4 \) \( \implies E_4=span\left\{\begin{pmatrix}2\\ 1\end{pmatrix}\right\} \)

\( \bullet\hspace{2mm} For\hspace{2mm}\lambda_2=-3 \) \( \implies E_{-3}=span\left\{\begin{pmatrix}-1\\ 3\end{pmatrix}\right\} \)

Therefore, \( A=PDP^{-1} \) where

\( P=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix} ,D=\begin{pmatrix}4&0\\ 0&-3\end{pmatrix} ,P^{-1}=\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)

(c) Write \( A^n \) in term of n.

since. \( A=PDP^{-1} \implies A^n=\bigg(PDP^{-1}\bigg)^n=PD^nP^{-1} \)

\( \implies A^n=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix}\begin{pmatrix}4^n&0\\ 0&\left(-3\right)^n\end{pmatrix}\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)

                \( =\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)

Therefore, \( A^n=\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)

 


Answer

Therefore. 

a). \( P(\lambda)=|A-\lambda I|=\bigg(\lambda-4\bigg)\bigg(\lambda+3\lambda\bigg) \)

b). \( A=PDP^{-1} \) where \( P=\begin{pmatrix}2&-3\\ 1&3\end{pmatrix} ,D=\begin{pmatrix}4&0\\ 0&-3\end{pmatrix} \),\( P^{-1}=\begin{pmatrix}\frac{3}{7}&\frac{1}{7}\\ -\frac{1}{7}&\frac{2}{7}\end{pmatrix} \)

c). \( A^n=\begin{pmatrix}\frac{3\cdot \:2^{2n+1}+3\left(-3\right)^n}{7}&\frac{2^{2n+1}-6\left(-3\right)^n}{7}\\ \frac{3\cdot \:4^n-3\left(-3\right)^n}{7}&\frac{4^n+6\left(-3\right)^n}{7}\end{pmatrix} \)

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