<s>[INST] Find $B=A^2+A$ knowing that $A^3=\begin{bmatrix}4&3\\-3&-2\end{bmatrix}$ 
Find $B=A^2+A$ knowing that $A^3=\begin{bmatrix}4&3\\-3&-2\end{bmatrix}$

Is there a way to solve this rather than just declaring a matrix $$A=\begin{bmatrix}a&b\\c&d\end{bmatrix}$$ and then trying to solve a system of cubic equations? My attempt:
$$A^3 -I_2 = \begin{bmatrix} 3 & 3\\ -3 & -3\end{bmatrix} = 9 \begin{bmatrix} 1 & 1\\ -1 & -1\end{bmatrix}$$
and 
$$B=\frac {A^3-I_2}{A-I_2}-I_2$$
$$(A-I_2)(A^2+A+I_2)=9\begin{bmatrix}1&1\\-1&-1\end{bmatrix}$$
But I get stuck here.
 [/INST] Denote the third root of unity as $w=e^{i2\pi/3}$.
Then any of the quantities $\rho=\{w,w^2,w^3=1\}\,$ satisfy $\,\rho^3=1,\,$ i.e. is a cube root of one.
Note that the matrix $X=A^3$ has a single eigenvalue of $\{\lambda=1\}$ with multiplicity two.
Since it is a $2\times 2$ matrix, any analytic function of $X$ (and its first derivative) can be written as linear polynomials
$$\eqalign{
f(X) &= \alpha_1X + \alpha_0I \cr
f'(X) &= \alpha_1I \cr
}$$
Evaluate the cube root function on the eigenvalues to calculate the polynomial coefficients 
$$\eqalign{
\alpha_1\lambda + \alpha_0 &= \lambda^{1/3} &\implies \alpha_1+\alpha_0 &= \rho \cr
\alpha_1 &= \tfrac{1}{3}\lambda^{-2/3} &\implies \alpha_1 &= \tfrac{\rho}{3} \cr
}$$
The matrix cube root is therefore
$$\eqalign{
A &= f(X) = \,\,\frac{\rho}{3}X + \frac{2\rho}{3}I \cr
}$$
and the matrix in question is
$$\eqalign{
B &= A^2 + A \cr
 &= (\tfrac{\rho}{3}X + \tfrac{2\rho}{3}I)^2 + (\tfrac{\rho}{3}X + \tfrac{2\rho}{3}I) \cr
 &= \tfrac{\rho^2}{9}(X^2+4X+4I) + \tfrac{\rho}{3}(X+2I) \cr
 &= \frac{1}{9}\Big(\rho^2X^2+(4\rho^2+3\rho)X+(4\rho^2+6\rho)I\Big) \cr\cr
}$$
This gives us 3 possible solutions for the $B$ matrix:
Setting $\rho=w^1=w\,$ yields
$$B_1=\frac{1}{9}\Big(w^2X^2+(w^2-3)X+(2w-4)I\Big)$$
Setting $\rho=w^2\,$ yields
$$B_2=\frac{1}{9}\Big(wX^2+(w-3)X+(2w^2-4)I\Big)$$
Setting $\rho=w^3=1\,$ yields
$$B_3=\frac{1}{9}\Big(X^2+7X+10I\Big)$$
Some properties, peculiar to roots-of-unity, were used to simplify the final expressions. Namely
$$\eqalign{
 w &= w^4 \cr
 0 &= 1+w+w^2 \cr
 0 &= 1+w^2+w^4 \cr\cr
}$$
Interestingly, these properties also mean that $\,\,B_1 + B_2 + B_3 = 0.$
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