Linear Algebra

If 2 by 2 Matrices Satisfy $A=AB-BA$, then $A^2$ is Zero Matrix

Problem 337

Let $A, B$ be complex $2\times 2$ matrices satisfying the relation
\[A=AB-BA.\]

Prove that $A^2=O$, where $O$ is the $2\times 2$ zero matrix.

Add to solve later

Hint.

Find the trace of $A$.
Use the Cayley-Hamilton theorem

Proof.

We first calculate the trace of the matrix $A$ as follows. We have
\begin{align*}
\tr(A)&=\tr(AB-BA)\\
&=\tr(AB)-\tr(BA)\\
&=\tr(AB)-\tr(AB)=0.
\end{align*}

Thus $\tr(A)=0$ and it follows from the Cayley-Hamilton theorem (see below) for the $2\times 2$ matrix $A$ that
\begin{align*}
O&=A^2-\tr(A)A+\det(A)I\\
&=A^2+\det(A)I,
\end{align*}
where $I$ is the $2\times 2$ identity matrix.

Thus, we obtain
\[A^2=-\det(A)I. \tag{*}\]

Next, we compute $A^2$ in two ways.
We have
\begin{align*}
A^2=A(AB-BA)=A^2B-ABA
\end{align*}
and
\begin{align*}
A^2=(AB-BA)A=ABA-BA^2.
\end{align*}
Adding these two, we have
\begin{align*}
2A^2&=A^2B-BA^2\\
& \stackrel{(*)}{=} (-\det(A)I)B-B(-\det(A)I)\\
&=-\det(A)B+\det(A)B=O.
\end{align*}

As a result, we obtain $A^2=O$. This completes the proof.

The Cayley-Hamilton theorem for a $2\times 2$ matrix

Let us add the proof of the fact we used in the proof about the Cayley-Hamilton theorem.
Let $A=\begin{bmatrix}
a & b\\
c& d
\end{bmatrix}$ be a $2\times 2$ matrix.

Then its characteristic polynomial is
\begin{align*}
p(x)&=\det(A-xI)\\
&=\begin{vmatrix}
a-x & b\\
c& d-x
\end{vmatrix}\\
&=(a-x)(d-x)-bc\\
&=x^2-(a+d)x+ad-bc\\
&=x^2-\tr(A)x+\det(A),
\end{align*}
since $\tr(A)=a+d$ and $\det(A)=ad-bc$.

The Cayley-Hamilton theorem says that the matrix $A$ satisfies its characteristic equation $p(x)=0$.
Namely we have
\[A^2-\tr(A)A+\det(A)I=O.\] This is the equality we used in the proof.

Variation

As a variation of this problem, consider the following problem.

Let $A, B$ be $2\times 2$ matrices satisfying $A=AB-BA$.
Then prove that $\det(A)=0$.

Add to solve later

More from my site

True or False: If $A, B$ are 2 by 2 Matrices such that $(AB)^2=O$, then $(BA)^2=O$ Let $A$ and $B$ be $2\times 2$ matrices such that $(AB)^2=O$, where $O$ is the $2\times 2$ zero matrix. Determine whether $(BA)^2$ must be $O$ as well. If so, prove it. If not, give a counter example. Proof. It is true that the matrix $(BA)^2$ must be the zero […]
If Two Matrices are Similar, then their Determinants are the Same Prove that if $A$ and $B$ are similar matrices, then their determinants are the same. Proof. Suppose that $A$ and $B$ are similar. Then there exists a nonsingular matrix $S$ such that \[S^{-1}AS=B\] by definition. Then we […]
Determine Whether Given Matrices are Similar (a) Is the matrix $A=\begin{bmatrix} 1 & 2\\ 0& 3 \end{bmatrix}$ similar to the matrix $B=\begin{bmatrix} 3 & 0\\ 1& 2 \end{bmatrix}$? (b) Is the matrix $A=\begin{bmatrix} 0 & 1\\ 5& 3 \end{bmatrix}$ similar to the matrix […]
Trace, Determinant, and Eigenvalue (Harvard University Exam Problem) (a) A $2 \times 2$ matrix $A$ satisfies $\tr(A^2)=5$ and $\tr(A)=3$. Find $\det(A)$. (b) A $2 \times 2$ matrix has two parallel columns and $\tr(A)=5$. Find $\tr(A^2)$. (c) A $2\times 2$ matrix $A$ has $\det(A)=5$ and positive integer eigenvalues. What is the trace of […]
$The Formula for the Inverse Matrix of $I+A$ for a $2\times 2$ Singular Matrix $A$$ The Formula for the Inverse Matrix of $I+A$ for a $2\times 2$ Singular Matrix $A$ Let $A$ be a singular $2\times 2$ matrix such that $\tr(A)\neq -1$ and let $I$ be the $2\times 2$ identity matrix. Then prove that the inverse matrix of the matrix $I+A$ is given by the following formula: \[(I+A)^{-1}=I-\frac{1}{1+\tr(A)}A.\] Using the formula, calculate […]
An Example of a Matrix that Cannot Be a Commutator Let $I$ be the $2\times 2$ identity matrix. Then prove that $-I$ cannot be a commutator $[A, B]:=ABA^{-1}B^{-1}$ for any $2\times 2$ matrices $A$ and $B$ with determinant $1$. Proof. Assume that $[A, B]=-I$. Then $ABA^{-1}B^{-1}=-I$ implies \[ABA^{-1}=-B. […]
How to Use the Cayley-Hamilton Theorem to Find the Inverse Matrix Find the inverse matrix of the $3\times 3$ matrix \[A=\begin{bmatrix} 7 & 2 & -2 \\ -6 &-1 &2 \\ 6 & 2 & -1 \end{bmatrix}\] using the Cayley-Hamilton theorem. Solution. To apply the Cayley-Hamilton theorem, we first determine the characteristic […]
Express the Eigenvalues of a 2 by 2 Matrix in Terms of the Trace and Determinant Let $A=\begin{bmatrix} a & b\\ c& d \end{bmatrix}$ be an $2\times 2$ matrix. Express the eigenvalues of $A$ in terms of the trace and the determinant of $A$. Solution. Recall the definitions of the trace and determinant of $A$: \[\tr(A)=a+d \text{ and } […]