Differences

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--- row-column_multiplication [2015/02/12 10:49] – created rupert
+++ row-column_multiplication [2015/02/17 13:08] (current) – rupert
@@ Line 1: / Line 1: @@
-If $a=\begin{bmatrix}a_1&a_2&\dots&a_n\end{bmatrix}$ is a $1\times n$ row vector and $b=\begin{bmatrix}b_1\\b_2\\\vdots\\b_n\end{bmatrix}$ is an $n\times 1$ column vector, then the product of $a$ and $b$ is defined to be
+If $a=\begin{bmatrix}a_1&a_2&\dots&a_n\end{bmatrix}$ is a $1\times n$ row vector and $b=\begin{bmatrix}b_1\\b_2\\\vdots\\b_n\end{bmatrix}$ is an $n\times 1$ column vector, then the **row-column product**, or simply the **product** of $a$ and $b$ is defined to be
 \[ ab=\begin{bmatrix}a_1&a_2&\dots&a_n\end{bmatrix}\begin{bmatrix}b_1\\b_2\\\vdots\\b_n\end{bmatrix}=a_1b_1+a_2b_2+\dots+a_nb_n.\]
+If we want to emphasize that we are multiplying in this way, we sometimes write $a\cdot b$ instead of $ab$.
+The product $ab$ of a $1\times m$ row vector $a$ with an $n\times 1$ column vector $b$ is undefined if $m\ne n$.