Solution IVLT.C20 Robert Beezer
(a) We will compute the kernel of $T$. Suppose that $a+bx+cx^2\in\krn{T}$. Then \begin{equation*} \begin{bmatrix}0 & 0 \\ 0 & 0\end{bmatrix} =\lt{T}{a+bx+cx^2} = \begin{bmatrix} a+2b-2c & 2a+2b \\ -a+b-4c & 3a+2b+2c \end{bmatrix} \end{equation*} and matrix equality (Theorem ME) yields the homogeneous system of four equations in three variables, \begin{align*} a+2b-2c&=0\\ 2a+2b&=0\\ -a+b-4c&=0\\ 3a+2b+2c&=0\\ \end{align*} The coefficient matrix of this system row-reduces as \begin{equation*} \begin{bmatrix} 1 & 2 & -2 \\ 2 & 2 & 0 \\ -1 & 1 & -4 \\ 3 & 2 & 2 \end{bmatrix} \rref \begin{bmatrix} \leading{1} & 0 & 2 \\ 0 & \leading{1} & -2 \\ 0 & 0 & 0 \\ 0 & 0 & 0 \end{bmatrix} \end{equation*} From the existence of nontrivial solutions to this system, we can infer nonzero polynomials in $\krn{T}$. By Theorem KILT we then know that $T$ is not injective.

(b) Since $3=\dimension{P_2}<\dimension{M_{22}}=4$, by Theorem SLTD $T$ is not surjective.

(c) Since $T$ is not surjective, it is not invertible by Theorem ILTIS.