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

Linear algebra problems and solutions

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.

 
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Hint.

  1. Find the trace of $A$.
  2. 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 $\det(A)=0$.


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