Single-entry Vector
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Single-entry Vector

In mathematics a single-entry matrix is a matrix where a single element is one and the rest of the elements are zero,[1][2] e.g.,

${\displaystyle \mathbf {J} ^{23}=\left[{\begin{matrix}0&0&0\\0&0&1\\0&0&0\end{matrix}}\right].}$

It is a specific type of a sparse matrix. The single-entry matrix can be regarded a row-selector when it is multiplied on the left side of the matrix, e.g.:

${\displaystyle \mathbf {J} ^{23}\mathbf {A} =\left[{\begin{matrix}0&0&0\\a_{31}&a_{32}&a_{33}\\0&0&0\end{matrix}}\right].}$

Alternatively, a column-selector when multiplied on the right side:

${\displaystyle \mathbf {A} \mathbf {J} ^{23}=\left[{\begin{matrix}0&0&a_{12}\\0&0&a_{22}\\0&0&a_{32}\end{matrix}}\right].}$

The name, single-entry matrix, is not common, but seen in a few works.[3]

A single-entry vector is a scaled standard unit vector.