Zero matrix

In mathematics, particularly linear algebra, a zero matrix or null matrix is a matrix with all its entries being zero.[1] Some examples of zero matrices are

${\displaystyle 0_{1,1}={\begin{bmatrix}0\end{bmatrix}},\ 0_{2,2}={\begin{bmatrix}0&0\\0&0\end{bmatrix}},\ 0_{2,3}={\begin{bmatrix}0&0&0\\0&0&0\end{bmatrix}}.\ }$

The set of m×n matrices with entries in a ring K forms a ring ${\displaystyle K_{m,n}\,}$. The zero matrix ${\displaystyle 0_{K_{m,n}}\,}$ in ${\displaystyle K_{m,n}\,}$ is the matrix with all entries equal to ${\displaystyle 0_{K}\,}$, where ${\displaystyle 0_{K}\,}$ is the additive identity in K.

${\displaystyle 0_{K_{m,n}}={\begin{bmatrix}0_{K}&0_{K}&\cdots &0_{K}\\0_{K}&0_{K}&\cdots &0_{K}\\\vdots &\vdots &\ddots &\vdots \\0_{K}&0_{K}&\cdots &0_{K}\end{bmatrix}}_{m\times n}}$

The zero matrix is the additive identity in ${\displaystyle K_{m,n}\,}$.[2] That is, for all ${\displaystyle A\in K_{m,n}\,}$ it satisfies

${\displaystyle 0_{K_{m,n}}+A=A+0_{K_{m,n}}=A.}$

There is exactly one zero matrix of any given size m×n having entries in a given ring, so when the context is clear one often refers to the zero matrix. In general the zero element of a ring is unique and typically denoted as 0 without any subscript indicating the parent ring. Hence the examples above represent zero matrices over any ring.

The zero matrix represents the linear transformation sending all vectors to the zero vector.[3]