Examples: matrix diagonalization
$2 \times 2$ matrix diagonalization
Let $A$ be a $2 \times 2$ matrix defined as
$$
\tag{1.1}
$$
Diagonalize $A$ and find the invertible matrix to diagonalize $A$.
Answer
● Preparation
For a square matrix $A$,
matrix diagonalization is to find a diagonal matrix $\Lambda$ satisfying
, where $P$ is an invertible matrix which diagonalizes $A$.
In the following, we find the diagonal matrix $\Lambda$
for the matrix $A$ in $(1.1)$,
and the invertive matrix $P$ that diagonalizes $A$.
It is known that
the diagonal elements of
a diagonalized matrix
are the eigenvalues of
the original matrix.
Therefore,
by obtaining eigenvalues of $A$
and arranging them in diagonal elements,
diagonalized matrix $\Lambda$ is obtained.
●
Derivation of diagonal matrix $\Lambda$
In order to obtain the eigenvalue $\Lambda$ of $A$,
we need to solve the characteristic equation
\begin{eqnarray}
\left| \lambda I - A \right| = 0
\end{eqnarray}
$$
\tag{1.2}
$$
, which is a polynomial equation in the variable (eigenvalue) $\lambda$.
Since the left-hand side is a
$2 \times 2$ determinant,
we have
Then the solutions of
$(1.2)$ are
By arranging these solutions in diagonal elements,
we obtain the diagonalized matrix $\Lambda$ as
$$
\tag{1.3}
$$
●
Derivation of invertible matrix that diagonalizes $A$
The invertible matrix $P$
diagonalizing the matrix $A$
is the matrix whose columun vectors are the eigenvectors of $A$.
Therefore, $P$ is obtained,
if the eigenvector for each eigenvalue of $A$ is obtained.
So, we will derive the eigenvectors of the eigenvalues of $A$ as follows.
Case $\lambda=5$
:
In this case,
the eigenvector $\mathbf{x}$ satisfies
Let $\mathbf{x}$ be
We have
Rearranging this equation,
we obtain
Therefore, the eigenvector is expressed as
,
where $x_{2}$ is an arbitrary value.
We set $x_2=1$ for convenience,
and obtain
$$
\tag{1.4}
$$
Case $\lambda=-2$
:
In this case, the eigenvector $\mathbf{x}$ satisfies
Let $\mathbf{x}$ be
We have
Rearranging this equation, we obtain
Therefore, the eigenvector is expressed as
where $x_2$ is an arbitrary value.
We set $x_2=1$ for convenience, and obtain
$$
\tag{1.5}
$$
Invertible matrix $P$
By $(1.4)$ and $(1.5)$,
we obtain the invertible matrix $P$ as
$$
\tag{1.6}
$$
● Check the answer
We will check
whether the matrix $P$ in equation $(1.6)$
actually diagonalizes the matrix $A$,
that is, whether $P$, $A$ and $\Lambda$ satisfy
To do that,
we need to derive the inverse matrix $P^{−1}$.
Derivation of $P^{-1}$
We will derive the inverse matrix $P^{−1}$
by
Gaussian elimination.
We define a matrix in which $P$ and the identity matrix $I$
are arranged side by side,
$$
\tag{1.7}
$$
and tramsform the left half matrix
to the identity matrix
by the elementary row operations:
As a result,
the matrix appearing in the right half
becomes the inverse matrix $P^{−1}$.
According to this method,
performing the elementary row operations
to the matrix $(1.7)$, we have
Therere we obtain
Check diagonalization
Now we can check the diagonalization as follows.
We see that
$P$ diagonalizes $A$.
$3 \times 3$ matrix diagonalization
Let $A$ be a $3 \times 3$ matrix defined as
$$
\tag{2.1}
$$
Diagonalize $A$ and find the invertible matrix to diagonalize $A$.
Answer
● Preparation
For a square matrix $A$,
matrix diagonalization is to find a diagonal matrix $\Lambda$ satisfying
, where $P$ is an invertible matrix which diagonalizes $A$.
In the following, we find the diagonal matrix $\Lambda$
for the matrix $A$ in $(2.1)$,
and the invertive matrix $P$ that diagonalizes $A$.
It is known that
the diagonal elements of
a diagonalized matrix
are the eigenvalues of
the original matrix.
Therefore,
by obtaining eigenvalues of $A$
and arranging them in diagonal elements,
diagonalized matrix $\Lambda$ is obtained.
●
Derivation of diagonal matrix $\Lambda$
In order to obtain the eigenvalue $\Lambda$ of $A$,
we need to solve the characteristic equation
$$
\tag{2.2}
$$
, which is a polynomial equation in the variable (eigenvalue) $\lambda$.
Since the left-hand side is a
$3 \times 3$ determinant,
we have
Then the solutions of $(2.2)$ are
By arranging these solutions in diagonal elements,
we obtain the diagonalized matrix $\Lambda$ as
$$
\tag{2.3}
$$
●
Derivation of invertible matrix that diagonalizes $A$
The invertible matrix $P$ that
diagonalizes the matrix $A$
is the matrix whose columun vectors are the eigenvectors of $A$.
Therefore, $P$ is obtained,
if the eigenvector for each eigenvalue of $A$ is obtained.
So, we will derive the eigenvectors of the eigenvalues of $A$ as follows.
Case $\lambda=-1$
:
In this case,
the eigenvector $\mathbf{x}$ satisfies
Let $\mathbf{x}$ be
We have
Rearranging this equation, we obtain
Therefore the eigenvector is expressed as
, where $x_{3}$ is an arbitrary value.
Here, we set $ x_ {3} = 1 $ for convenience, and obtain
$$
\tag{2.4}
$$
Case $\lambda=1$
:
In this case,
the eigenvector $\mathbf{x}$ satisfies
Let $\mathbf{x}$ be
We have
Rearranging this equation, we obtain
Therefore the eigenvector is expressed as
, where $x_{3}$ is an arbitrary value.
Here, we set $ x_ {3} = 1 $ for convenience, and obtain
$$
\tag{2.5}
$$
Case $\lambda=2$
:
In this case,
the eigenvector $\mathbf{x}$ satisfies
Let $\mathbf{x}$ be
We have
Rearranging this equation, we obtain
Therefore the eigenvector is expressed as
, where $x_{3}$ is an arbitrary value.
Here, we set $ x_ {3} = 1 $ for convenience, and obtain
$$
\tag{2.6}
$$
Invertible matrix $P$
By (2.4), (2.5) and (2.6),
we obtain the invertible matrix $P$ as
$$
\tag{2.7}
$$
● Check the answer
We will check
whether the matrix $P$ in equation $(2.7)$
actually diagonalizes the matrix $A$,
that is, whether $P$, $A$ and $\Lambda$ satisfy
To do that,
we need to derive the inverse matrix $P^{−1}$.
Derivation of $P^{-1}$
We will derive the inverse matrix $P^{−1}$
by
Gaussian elimination.
We define a matrix in which $P$ and the identity matrix $I$
are arranged side by side,
$$
\tag{2.8}
$$
and tramsform
the left half matrix to the identity matrix
by the elementary row operations:
As a result,
the matrix appearing in the right half
becomes the inverse matrix $A^{−1}$.
According to this method,
performing the elementary row operations to the matrix $(2.8)$,
we have
Therere we obtain
Check diagonalization
Now we can check the diagonalization as follows.
We see that $P$ diagonalizes $A$.