Matrix Product (Conventional)/Examples/Column Matrix All 0 except for One 1
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Example of (Conventional) Matrix Product
Let $\mathbf A$ be a matrix of order $m \times n$.
For $1 \le i \le n$, let $\mathbf e_i$ be the column matrix of order $n \times 1$ defined as:
- $e_k = \delta_{k i}$
where:
- $e_k$ is the element of $\mathbf e_i$ whose indices are $\tuple {k, 1}$
- $\delta_{k i}$ denotes the Kronecker delta.
Then $\mathbf A \mathbf e_i$ is the column matrix which is equal to the $i$th column of $\mathbf A$.
Proof
Let $\mathbf B = \mathbf A \mathbf e_i$
By definition of matrix product, $\mathbf B$ is a matrix of order $m \times 1$.
Let $b_j$ denote the element of $\mathbf B$ whose indices are $\tuple {j, 1}$.
We have:
\(\ds b_j\) | \(=\) | \(\ds \sum_{k \mathop = 1}^n a_{j k} e_k\) | Definition of Matrix Product (Conventional) | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^n a_{j k} \delta_{k i}\) | Definition of $\mathbf e_i$ | |||||||||||
\(\ds \) | \(=\) | \(\ds a_{j i}\) |
Hence the result.
$\blacksquare$
Sources
- 1998: Richard Kaye and Robert Wilson: Linear Algebra ... (previous) ... (next): Part $\text I$: Matrices and vector spaces: $1$ Matrices: Exercises: $1.9$