# Unitary Matrices and Hermitian Matrices

Recall that the conjugate of a complex number is . The conjugate of is denoted or .

In this section, I'll use for complex conjugation of numbers of matrices. I want to use to denote an operation on matrices, the conjugate transpose.

Thus,

Complex conjugation satisfies the following properties:

(a) If , then if and only if z is a real number.

(b) If , then

(c) If , then

The proofs are easy; just write out the complex numbers (e.g. and ) and compute.

The conjugate of a matrix A is the matrix obtained by conjugating each element: That is,

You can check that if A and B are matrices and , then

You can prove these results by looking at individual elements of the matrices and using the properties of conjugation of numbers given above.

Definition. If A is a complex matrix, is the conjugate transpose of A:

Note that the conjugation and transposition can be done in either order: That is, . To see this, consider the element of the matrices:

Example. If

Since the complex conjugate of a real number is the real number, if B is a real matrix, then .

Remark. Most people call the adjoint of A --- though, unfortunately, the word "adjoint" has already been used for the transpose of the matrix of cofactors in the determinant formula for . (Sometimes people try to get around this by using the term "classical adjoint" to refer to the transpose of the matrix of cofactors.) In modern mathematics, the word "adjoint" refers to a property of that I'll prove below. This property generalizes to other things which you might see in more advanced courses.

The operation is sometimes called the Hermitian --- but this has always sounded ugly to me, so I won't use this terminology.

Since this is an introduction to linear algebra, I'll usually refer to as the conjugate transpose, which at least has the virtue of saying what the thing is.

Proposition. Let U and V be complex matrices, and let .

(a) .

(b) .

(c) .

(d) If , their dot product is given by

Proof. I'll prove (a), (c), and (d).

For (a), I use the fact noted above that and can be done in either order, along with the facts that

I have

This proves (a).

For (c), I have

For (d), recall that the dot product of complex vectors and is

Notice that you take the complex conjugates of the components of v before multiplying!

This can be expressed as the matrix multiplication

Example.

It's a common notational abuse to write the number " " instead of writing it as a matrix " ".

There are two points about the equation which might be confusing. First, why is it necessary to conjugate and transpose v? The reason for the conjugation goes back to the need for inner products to be positive definite (so is a nonnegative real number).

The reason for the transpose is that I'm using the convention that vectors are column vectors. So if u and v are n-dimensional column vectors and I want the product to be a number --- i.e. a matrix --- I have to multiply an n-dimensional row vector ( ) and an n-dimensional column vector ( ). To get the row vector, I have to transpose the column vector.

Finally, why do u and v switch places in going from the left side to the right side? The reason you write instead of is because inner products are defined to be linear in the first variable. If you use you get a product which is linear in the second variable.

Of course, none of this makes any difference if you're dealing with real numbers. So if x and y are vectors in , you can write

Definition. A complex matrix U is unitary if .

Notice that if U happens to be a real matrix, , and the equation says --- that is, U is orthogonal. In other words, unitary is the complex analog of orthogonal.

By the same kind of argument I gave for orthogonal matrices, implies --- that is, is .

Proposition. Let U be a unitary matrix.

(a) U preserves inner products: . Consequently, it also preserves lengths: .

(b) An eigenvalue of U must have length 1.

(c) The columns of a unitary matrix form an orthonormal set.

Proof. (a)

Since U preserves inner products, it also preserves lengths of vectors, and the angles between them. For example,

(b) Suppose x is an eigenvector corresponding to the eigenvalue of U. Then , so

But U preserves lengths, so , and hence .

(c) Suppose

Then means

Here is the complex conjugate of the column , transposed to make it a row vector. If you look at the dot products of the rows of and the columns of U, and note that the result is I, you see that the equation above exactly expresses the fact that the columns of U are orthonormal.

For example, take the first row . Its product with the columns , , and so on give the first row of the identity matrix, so

This says that has length 1 and is perpendicular to the other columns. Similar statements hold for , ..., .

Example. Find c and d so that the following matrix is unitary:

I want the columns to be orthogonal, so their complex dot product should be 0. First, I'll find a vector that is orthogonal to the first column. I may ignore the factor of ; I need

This gives

I may take and . Then

So I need to divide each of a and b by to get a unit vector. Thus,

Proposition. ( Adjointness) let and let . Then

Proof.

Remark. If is any inner product on a vector space V and is a linear transformation, the adjoint of T is the linear transformation which satisfies

(This definition assumes that there is such a transformation.) This explains why, in the special case of the complex inner product, the matrix is called the adjoint. It also explains the term self-adjoint in the next definition.

Corollary. ( Adjointness) let and let . Then

Proof. This follows from adjointness in the complex case, because for a real matrix.

Definition. An complex matrix A is Hermitian (or self-adjoint) if .

Note that a Hermitian matrix is automatically square.

For real matrices, , and the definition above is just the definition of a symmetric matrix.

Example. Here are examples of Hermitian matrices:

It is no accident that the diagonal entries are real numbers --- see the result that follows.

Here's a table of the correspondences between the real and complex cases:

Proposition. Let A be a Hermitian matrix.

(a) The diagonal elements of A are real numbers, and elements on opposite sides of the main diagonal are conjugates.

(b) The eigenvalues of a Hermitian matrix are real numbers.

(c) Eigenvectors of A corresponding to different eigenvalues are orthogonal.

Proof. (a) Since , I have . This shows that elements on opposite sides of the main diagonal are conjugates.

Taking , I have

But a complex number is equal to its conjugate if and only if it's a real number, so is real.

(b) Suppose A is Hermitian and is an eigenvalue of A with eigenvector v. Then

Therefore, --- but a number that equals its complex conjugate must be real.

(c) Suppose is an eigenvalue of A with eigenvector u and is an eigenvalue of A with eigenvector v. Then

implies , so if the eigenvalues are different, then .

Since real symmetric matrices are Hermitian, the previous results apply to them as well. I'll restate the previous result for the case of a symmetric matrix.

Corollary. Let A be a symmetric matrix.

(a) The elements on opposite sides of the main diagonal are equal.

(b) The eigenvalues of a symmetric matrix are real numbers.

(c) Eigenvectors of A corresponding to different eigenvalues are orthogonal.

Example. Consider the symmetric matrix

The characteristic polynomial is .

Note that the eigenvalues are real numbers.

For , an eigenvector is .

For , an eigenvector is .

Since , the eigenvectors are orthogonal.

Example. A real symmetric matrix A has eigenvalues 1 and 3.

is an eigenvector corresponding to the eigenvalue 1.

(a) Find an eigenvector corresponding to the eigenvalue 3.

Let be an eigenvector corresponding to the eigenvalue 3.

Since eigenvectors for different eigenvalues of a symmetric matrix must be orthogonal, I have

So, for example, is a solution.

(b) Find A.

From (a), a diagonalizing matrix and the corresponding diagonal matrix are

Now , so

Note that the result is indeed symmetric.

Example. Let , and consider the Hermitian matrix

Compute the characteristic polynomial of A, and show directly that the eigenvalues must be real numbers.

The discriminant is

Since this is a sum of squares, it can't be negative. Hence, the roots of the characteristic polynomial --- the eigenvalues --- must be real numbers.