Matrix Mathematical Notation I am trying to work out the mathematical notation for combining the columns of two matrices, $$A=\begin{pmatrix}1 & 3 & 5 \\ 2 & 4 & 1 \\ 3 & 7 & 9\end{pmatrix}$$ and $$B=\begin{pmatrix}4 & 4 & 3 \\ 9 & 10 & 11 \\ 12 & 15 & 13\end{pmatrix},$$ to form the new matrix $$C=\begin{pmatrix}1 & 4 & 3\\ 2 & 10 & 11 \\ 3 & 15 & 13\end{pmatrix}.$$  $C$ is a matrix which is made up of the first column of $A$ and the last two columns of $B$.  The problem I have is expressing $C$ in terms of $A$ and $B$ using appropriate mathematical notation, I can code it, I just don't know the notation for it!  Any suggestions?

Describing the columns of the matrices as vectors, you can also say
$$
\begin{align*}
A &= \begin{bmatrix}\mathbf{a}_1 & \mathbf{a}_2 & \mathbf{a}_3 \end{bmatrix}, \\
B &= \begin{bmatrix}\mathbf{b}_1 & \mathbf{b}_2 & \mathbf{b}_3 \end{bmatrix}, \\
C &= \begin{bmatrix}\mathbf{a}_1 & \mathbf{b}_2 & \mathbf{b}_3 \end{bmatrix}.
\end{align*}
$$
where the reader will instantly see what is happening. But of course this is not a "mathematical operation" to obtain $C$ from $A$ and $B$ as in the other answers.
