Dot Product 2x2 Matrix


by Owen-
Tags: matrix, product
Owen-
Owen- is offline
#1
Apr29-13, 10:27 AM
P: 40
This seems like a very basic question that I should know the answer to, but in my image processing class, my teacher explained that a basis set of images(matrices) are orthonormal.

He said that the DOT product between two basis images (in this case two 2x2 matrices) is 0. so, for example

\begin{equation}
\begin{bmatrix}
a & b\\
c & d
\end{bmatrix}
\cdot
\begin{bmatrix}
e & f\\
g & h
\end{bmatrix}
=0
\end{equation}

I don't understand how this can be. I always thought it gave another matrix, and not a direct value:
\begin{equation}
\begin{bmatrix}
a & b\\
c & d
\end{bmatrix}
\cdot
\begin{bmatrix}
e & f\\
g & h
\end{bmatrix}
=
\begin{bmatrix}
ae+bg & af+bh\\
ce+dg & cf+dh
\end{bmatrix}
\end{equation}

Can someone help me out? It would be unbelieveably helpful,
Thanks!
Owen.
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pwsnafu
pwsnafu is offline
#2
Apr29-13, 11:05 AM
Sci Advisor
P: 779
The only possibility I can think of is to take a 2x2 matrix and write it out in the form ##a e_{11} + b e_{12} + c e_{21} + d e_{22}##, ie as a four dimensional vector space. Then the e's form an orthonormal basis.
D H
D H is offline
#3
Apr29-13, 11:30 AM
Mentor
P: 14,477
Quote Quote by Owen- View Post
I don't understand how this can be. I always thought it gave another matrix, and not a direct value:
\begin{equation}
\begin{bmatrix}
a & b\\
c & d
\end{bmatrix}
\cdot
\begin{bmatrix}
e & f\\
g & h
\end{bmatrix}
=
\begin{bmatrix}
ae+bg & af+bh\\
ce+dg & cf+dh
\end{bmatrix}
\end{equation}

Can someone help me out? It would be unbelieveably helpful,
Thanks!
Owen.
That's the matrix product, not the dot product. A dot product (inner product) is a scalar. Always. For matrices, the typical definition of the dot product is the Frobenius inner product. Simply compute as if the matrix was a vector. For real matrices,

\begin{equation}
A\cdot B \equiv \sum_i \sum_j A_{ij} B_{ij}
\end{equation}
For your pair of 2x2 matrices,
\begin{equation}
\begin{bmatrix}
a & b\\
c & d
\end{bmatrix}
\cdot
\begin{bmatrix}
e & f\\
g & h
\end{bmatrix}
= ae + bf + cg + dh\end{equation}

Owen-
Owen- is offline
#4
Apr29-13, 12:02 PM
P: 40

Dot Product 2x2 Matrix


Perfect thanks a lot!


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