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# ishermitian

Determine if matrix is Hermitian or skew-Hermitian

## Description

example

tf = ishermitian(A) returns logical 1 (true) if square matrix A is Hermitian; otherwise, it returns logical 0 (false).

example

tf = ishermitian(A,skewOption) specifies the type of the test. Specify skewOption as 'skew' to determine if A is skew-Hermitian.

## Examples

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### Test if Symmetric Matrix Is Hermitian

Create a 3-by-3 matrix.

`A = [1 0 1i; 0 1 0; 1i 0 1]`
```A =

1.0000 + 0.0000i   0.0000 + 0.0000i   0.0000 + 1.0000i
0.0000 + 0.0000i   1.0000 + 0.0000i   0.0000 + 0.0000i
0.0000 + 1.0000i   0.0000 + 0.0000i   1.0000 + 0.0000i```

The matrix is symmetric with respect to its real-valued diagonal.

Test whether the matrix is Hermitian.

`tf = ishermitian(A)`
```tf =

0```

The result is logical 0 (false) because A is not Hermitian. In this case, A is equal to its transpose, A.', but not its complex conjugate transpose, A'.

Change the element in A(3,1) to be -1i.

`A(3,1) = -1i;`

Determine if the modified matrix is Hermitian.

```tf = ishermitian(A)
```
```tf =

1```

The matrix, A, is now Hermitian because it is equal to its complex conjugate transpose, A'.

### Test if Matrix Is Skew-Hermitian

Create a 3-by-3 matrix.

```A = [-1i -1 1-i;1 -1i -1;-1-i 1 -1i]
```
```A =

0.0000 - 1.0000i  -1.0000 + 0.0000i   1.0000 - 1.0000i
1.0000 + 0.0000i   0.0000 - 1.0000i  -1.0000 + 0.0000i
-1.0000 - 1.0000i   1.0000 + 0.0000i   0.0000 - 1.0000i```

The matrix has pure imaginary numbers on the main diagonal.

Specify skewOption as 'skew' to determine whether the matrix is skew-Hermitian.

`tf = ishermitian(A,'skew')`
```tf =

1```

The matrix, A, is skew-Hermitian since it is equal to the negation of its complex conjugate transpose, -A'.

## Input Arguments

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### A — Input matrixnumeric matrix

Input matrix, specified as a numeric matrix. If A is not square, then ishermitian returns logical 0 (false).

Data Types: single | double
Complex Number Support: Yes

### skewOption — Test type'nonskew' (default) | 'skew'

Test type, specified as 'nonskew' (default) or 'skew'. Specify 'skew' to test whether A is skew-Hermitian. Specifying ishermitian(A,'nonskew') is equivalent to ishermitian(A).

Data Types: char

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### Hermitian Matrix

• A square matrix, A, is Hermitian if it is equal to its complex conjugate transpose, A = A'.

In terms of the matrix elements, this means that

${a}_{i,\text{\hspace{0.17em}}j}={\overline{a}}_{j,\text{\hspace{0.17em}}i}\text{\hspace{0.17em}}\text{\hspace{0.17em}}.$

• The entries on the diagonal of a Hermitian matrix are always real. Since real matrices are unaffected by complex conjugation, a real matrix that is symmetric is also Hermitian. For example, the matrix

$A=\left[\begin{array}{cc}\begin{array}{c}1\\ 0\end{array}& \begin{array}{cc}\begin{array}{c}0\\ 2\end{array}& \begin{array}{c}1\\ 0\end{array}\end{array}\\ 1& \begin{array}{cc}0& 1\end{array}\end{array}\right]$

is both symmetric and Hermitian.

• The eigenvalues of a Hermitian matrix are real.

### Skew-Hermitian Matrix

• A square matrix, A, is skew-Hermitian if it is equal to the negation of its complex conjugate transpose, A = -A'.

In terms of the matrix elements, this means that

${a}_{i,\text{\hspace{0.17em}}j}=-{\overline{a}}_{j,\text{\hspace{0.17em}}i}\text{\hspace{0.17em}}\text{\hspace{0.17em}}.$

• The entries on the diagonal of a skew-Hermitian matrix are always pure imaginary or zero. Since real matrices are unaffected by complex conjugation, a real matrix that is skew-symmetric is also skew-Hermitian. For example, the matrix

$A=\left[\begin{array}{cc}0& -1\\ 1& \text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}0\end{array}\right]$

is both skew-Hermitian and skew-symmetric.

• The eigenvalues of a skew-Hermitian matrix are purely imaginary or zero.