Question #243422

Let A be a 2 x 2 matrix. Show that some non-trivial linear combination of A^4, A^3, A^2, A. and I2 is equal 0. Generalize to n x n matrices. Note that I2 is 2 x 2 identity matrix.


1
Expert's answer
2021-09-30T00:30:14-0400

Let A=I=[1001]A=I=\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix} (2×2 matrix)(2\times 2 \ matrix)

A4=A3=A2=I=[1001]\therefore A^4=A^3=A^2=I=\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}

Linear Combination:

aA4+bA3+cA2+dA+eI2=0aA^4+bA^3+cA^2+dA+eI^2=0

(a+b+c+d+e)[1001]=0\Rightarrow (a+b+c+d+e)\begin{bmatrix} 1 & 0 \\ 0 & 1 \end{bmatrix}=0

a+b+c+d+e=0\therefore a+b+c+d+e=0


Now, solving for (n×n matrix):(n\times n \ matrix):


Let A=In=[1000000.......00100000.......00010000.......0....0000000.......1]A= I_n=\begin{bmatrix} 1 & 0 & 0& 0& 0& 0& 0 .......& 0\\ 0 & 1 & 0& 0& 0& 0& 0.......& 0\\ 0 & 0 & 1& 0& 0& 0& 0.......& 0\\ . \\ . \\ . \\ . 0 & 0 & 0& 0& 0& 0& 0.......& 1\\ \end{bmatrix}

A4=A3=A2=A=In=[1000000.......00100000.......00010000.......0....0000000.......1]\therefore A^4=A^3=A^2=A= I_n=\begin{bmatrix} 1 & 0 & 0& 0& 0& 0& 0 .......& 0\\ 0 & 1 & 0& 0& 0& 0& 0.......& 0\\ 0 & 0 & 1& 0& 0& 0& 0.......& 0\\ . \\ . \\ . \\ . 0 & 0 & 0& 0& 0& 0& 0.......& 1\\ \end{bmatrix}

Linear Combination:

aA4+bA3+cA2+dA+eIn2=0aA^4+bA^3+cA^2+dA+eI_n^2=0

(a+b+c+d+e)[1000000.......00100000.......00010000.......0....0000000.......1]=0\Rightarrow (a+b+c+d+e)\begin{bmatrix} 1 & 0 & 0& 0& 0& 0& 0 .......& 0\\ 0 & 1 & 0& 0& 0& 0& 0.......& 0\\ 0 & 0 & 1& 0& 0& 0& 0.......& 0\\ . \\ . \\ . \\ . 0 & 0 & 0& 0& 0& 0& 0.......& 1\\ \end{bmatrix}=0

a+b+c+d+e=0\therefore a+b+c+d+e=0

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