More about Eigen Structure and Diagonalization

Remarks: Let $A$ and $B$ be $n \times n$ matrices.

1. We said that if $A$ and $B$ are similar, then they have the same eigenvalues. In particular, if $A$ is diagonalizable, then $A$ is similar to a diagonal matrix $D$. Thus, $A$ and $D$ have the same eigenvalues which means the diagonal elements of $D$ are the eigenvalues of $A$. Now if $A$ is diagonalizable, then there exists a nonsingular matrix $P$ such that $P^{-1}AP = D$. In this case, we say $P$ diagonalizes $A$ or $A$ is diagonalizable via $P$. Now after explaining the relationship between $A$ and $D$ (they have the same eigenvalues), the questions that arise are:
   1. What is the relationship between $A$ and $P$?

   2. Are $P$ and $D$ unique?

   The answer to this first is that the columns of $P$ are eigenvectors of $A$. The answer to the second is $P$ is not unique and $D$ is not necessarily unique. In fact, if you multiply $P$ by any nonzero number, then the resulting matrix will diagonalize $A$. Also, if you change the order of the columns of $P$, then the resulting matrix will diagonalize $A$. If you do so, $D$ also may change (will have the same elements as the original, but the location of these elements may change), which means $D$ is not necessarily unique.

   Remark: To diagonalize an $n \times n$ matrix $A$ that has $n$ linearly independent eigenvectors, find $n$ linearly independent eigenvectors $x_1$, $x_2$, $\cdots$, $x_n$, of $A$. Then form the matrix $P = [x_1 ~ x_2 ~ \cdots ~ x_n]$. Now $P^{-1}AP = D$, where $D$ is a diagonal matrix whose diagonal elements are the eigenvalues of $A$ corresponding to the eigenvalues of $x_1$, $x_2$, $\cdots$, $x_n$.

2. Now recall that the eigenvalues of Hermitian matrices and the diagonal elements are real and the eigenvalues of skew-Hermitian matrices and the diagonal elements have zero real parts. Also, recall that if $M$ is unitary, then $\vert\det(M)\vert = 1$ (note $\det(M)$ can be complex), $M$ is normal, $\Vert Mx\Vert _2 = \Vert x\Vert _2$, $\forall x \in \mathbb{C}^n$, and if $\lambda$ is an eigenvalue of $M$, then $\vert\lambda\vert=1$. Moreover, any two distinct columns of $M$ are orthogonal (i.e. $M(*,i) \cdot M(*,j) = M(*,i)^H M(*,j)=0$, when $i \neq j$, and each one of them is a unit vector). Here are more facts about these matrices:
   1. (Schur's Theorem) If $A$ is an $n \times n$ matrix, then there exists a unitary matrix $M$ such that $M^HAM = U$, where $U$ is upper-triangular. Moreover, $A$ and $U$ have the same eigenvalues.

   2. From the above, note that we can write $A=MUM^H$ (proof: exercise). This is called the Schur decomposition of $A$ or Schur normal form.

   3. From Schur's theorem: If $A$ is an $n \times n$ hermitian or a skew-Hermitian matrix, then there exists a unitary matrix $M$ such that $M^HAM = D$, where $D$ is diagonal. Thus, $A$ is diagonalizable, and we say in this case $A$ is unitarily diagonalizable.

      Proof of the Hermitian Case: By Schur's Theorem, there exists a unitary matrix $M$ and an upper-triangular matrix $U$ such that $M^HAM = U$. Now take the Hermitian transpose of both sides to get $M^H A^H (M^H)^H = U^H$. Thus, $M^H A M = U^H$. But, $M^HAM = U$ also. Therefore, $U^H = U$, which implies $U$ is diagonal. The skew-Herimitian case is similar.

   4. $U$ in Schur's decomposition is diagonal iff $A$ is normal. Moreover, when $U$ is normal, the rows of $M$ are eigenvetors of $A$. Thus, $A$ is unitarily diagonalizable iff $A$ is normal. Moerover, $A$ is normal iff $A$ has a complete orthonormal set of eigenvectors.

   5. From the previous part, if you take $A$ to be real symmetric, then there exists an orthogonal matrix $M$ such that $M^T AM = D$, where $D$ is diagonal. Thus, $A$ is diagonalizable, and we say in this case $A$ is orthogonally diagonalizable (proof: exercise). The eigenvalues of a real symmetric matrix are real and eigenvectors are real. Also, eigenvectors corresponding to different eigenvalues are orthogonal.

   6. If $A$ is Herimitian, then eigenvectors that correspond to different eigenvalues are orthogonal.

      Proof: Let $(\lambda_1,x)$ and $(\lambda_2,y)$ be two eigenpairs of $A$ where $\lambda_1 \neq \lambda_2$. We have to prove that $x^H y = 0$. Now consider

      $$(Ax)^H y = x^H A^H y = x^H A y = \lambda_2 x^H y.$$
      $$(Ax)^H y = (y^H A x)^H = (\lambda_1 y^H x)^H = \lambda_1 x^H y.$$
      Therefore, $\lambda_1 x^H y = \lambda_2 x^H y$, which implies $\lambda_1 x^H y - \lambda_2 x^H y = (\lambda_1 - \lambda_2) x^H y = 0$, . Since $\lambda_1 \neq \lambda_2$, then $x^H y = 0$.

3. $A$ is orthogonaly diagonalizable iff $A$ has $n$ orthonormal eigenvectors iff $A$ is real symmetric.

4. (Cayley-Hamilton Theorem) Every matrix satisfies its characteristic equation.

Transforming Complex Hermitian Eigenvalue Problems to Real Ones

Let $C = A + i B$ be a complex Hermitian matrix, where $A$ and $B$ are real $n \times n$ matrices and let $(\lambda,z=x+iy)$ be an eigenpair of $C$, where $x$ and $y$ are in $\mathbb{R}^n$ (recall that $\lambda$ is real because $C$ is Hermitian). Now, $(A+iB)(x+iy)=\lambda (x+iy)$ if and only if

$$\left[\begin{array}{cc} A & -B\\ B & A\end{array}\right]\left[\b... ...\ y\end{array}\right]=\lambda \left[\begin{array}{c} x\\ y\end{array}\right].$$

Note that since $C$ is Hermitian, then $A$ is symmetric and $B$ is skew-symmetric. Hence, the matrix $\left[\begin{array}{cc} A & -B\\ B & A\end{array}\right]$ is real symmetric. Thus, we managed to reduce a complex Hermitian eigenvalue problem of order $n$ to a real symmetric eigenvalue problem of order $2n$.

The companion Matrix

Let $A$ be the $n \times n$ matrix such that $a(k,k+1)=1$, $k=1,2, \cdots, n-1$, and $a_(n,k)=-c_{k-1}$, $k=1,2, \cdots, n$. By expanding the determinant of $A-\lambda I$ across the last row, you'll find out that the characterestic polynomial of $A$ is

$$p(\lambda)=(-1)^n \left(x^n + \sum_{k=1}^{n} c_{k-1} x^{k-1} \right).$$

The matrix $A$ is called the companion matrix of the polynomial $p(\lambda)$. The companion matrix is sometimes defined to be the traspose of the matrix above and it satisfies the following: $A e_k = e_{k+1}$, $k=1,2, \cdots, n-1$, and $Ae_n = [-c_0 ~ -c_1 ~ \cdots ~ -c_{n-1}]^T$.

Definition: The spectral radius of an $n \times n$ matrix $A$, denoted $\rho(A)$, is the maximum eigenvalue of $A$ in magnitude; i.e. if the eigenvalues of $A$ are $\lambda_1,$ $\lambda_2$, $\cdots$, $\lambda_n$, then $\rho(A) =$   max$_i \vert\lambda_i\vert$.

Definition: Let $A$ be an $m \times n$ matrix and let $B$ be the matrix such that $b_{ij} = \vert a_{ij}\vert$.

1. The one-norm of $A$, denoted $\Vert A\Vert _1$, is the maximum column sum of $B$.

2. The $\infty$-norm, denoted $\Vert A\Vert _{\infty}$, of $A$ is the maximum rwo sum of $B$.

3. The two-norm (or spectral norm) of $A$, denoted $\Vert A\Vert _2$, is the non-negative square root of $\rho(A^TA)$. Note that $A^T A$ is square and symmetric.

4. The Frobenius norm of $A$, denoted $\Vert A\Vert _F$, is $\sqrt{\sum_{i=1}^m \sum_{j=1}^n \vert a_{ij}\vert^2}$.

Remark: There are properties and equivalent definitions for matrix norms, but we don't have time to go over that.

Theorem (Gerschgorin): Let $A$ be an $n \times n$ matrix and define the disks

$$D_k = \{z \in \mathbb{C}~\vert~ \vert z - a_{kk}\vert \leq \sum_{j \neq k} \vert a_{kj}\vert,~k=1,~2,~\cdots,~n.$$

If $\lambda$ is an eigenvalue of $A$, then $\lambda$ is located in $\bigcup_{k=1}^n D_k$.

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