The Unapologetic Mathematician

Mathematics for the interested outsider

Unitary and Orthogonal Matrices and Orthonormal Bases

I almost forgot to throw in this little observation about unitary and orthogonal matrices that will come in handy.

Let’s say we’ve got a unitary transformation U and an orthonormal basis \left\{e_i\right\}_{i=1}^n. We can write down the matrix as before

\displaystyle\begin{pmatrix}u_{1,1}&\cdots&u_{1,n}\\\vdots&\ddots&\vdots\\u_{n,1}&\cdots&u_{n,n}\end{pmatrix}

Now, each column is a vector. In particular, it’s the result of transforming a basis vector e_i by U.

\displaystyle U(e_i)=u_{1,i}e_1+\dots+u_{n,i}e_n

What do these vectors have to do with each other? Well, let’s take their inner products and find out.

\displaystyle\langle U(e_i),U(e_j)\rangle=\langle e_i,e_j\rangle=\delta_{i,j}

since U preserves the inner product. That is the collection of columns of the matrix of U form another orthonormal basis.

On the other hand, what if we have in mind some other orthonormal basis \left\{f_j\right\}_{j=1}^n. We can write each of these vectors out in terms of the original basis

\displaystyle f_j=a_{1,j}e_1+\dots+a_{n,j}e_n

and even get a change-of-basis transformation (like we did for general linear transformations) A defined by

\displaystyle A(e_j)=f_j=a_{1,j}e_1+\dots+a_{n,j}e_n

so the a_{i,j} are the matrix entries for A with respect to the basis \left\{e_i\right\}. This transformation A will then be unitary.

Indeed, take arbitrary vectors v=v^ie_i and w=w^je_j. Their inner product is

\displaystyle\langle v,w\rangle=\langle v^ie_i,w^je_j\rangle=\overline{v^i}w^j\langle e_i,e_j\rangle=\overline{v^i}w^j\delta_{i,j}

On the other hand, after acting by A we find

\displaystyle\langle A(v),A(w)\rangle=\langle v^iA(e_i),w^jA(e_j)\rangle=\overline{v^i}w^j\langle f_i,f_j\rangle=\overline{v^i}w^j\delta_{i,j}

since the basis \left\{f_j\right\} is orthonormal as well.

To sum up: with respect to an orthonormal basis, the columns of a unitary matrix form another orthonormal basis. Conversely, writing any other orthonormal basis in terms of the original basis and using these coefficients as the columns of a matrix gives a unitary matrix. The same holds true for orthogonal matrices, with similar reasoning all the way through. And both of these are parallel to the situation for general linear transformations: the columns of an invertible matrix with respect to any basis form another basis, and conversely.

About these ads

August 7, 2009 - Posted by | Algebra, Linear Algebra

3 Comments »

  1. [...] Now if we have an arbitrary orthonormal basis — say is a transformation on with the standard basis already floating around — we may want to work with the matrix of with respect to this basis. If this were our basis of eigenvectors, would have the diagonal matrix . But we may not be so lucky. Still, we can perform a change of basis using the basis of eigenvectors to fill in the columns of the change-of-basis matrix. And since we’re going from one orthonormal basis to another, this will be unitary! [...]

    Pingback by The Complex Spectral Theorem « The Unapologetic Mathematician | August 10, 2009 | Reply

  2. [...] already floating around for , we can use this new basis to perform a change of basis, which will be orthogonal (not unitary in this case). That is, we can write the matrix of any self-adjoint transformation as [...]

    Pingback by The Real Spectral Theorem « The Unapologetic Mathematician | August 14, 2009 | Reply

  3. [...] numbers) we find that they’re orthonormal. But this means that the modified table is a unitary matrix, and thus its columns are orthonormal as well. We conclude [...]

    Pingback by The Character Table as Change of Basis « The Unapologetic Mathematician | November 22, 2010 | Reply


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 386 other followers

%d bloggers like this: