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In mathematics, particularly linear algebra, a zero matrix or null matrix is a matrix all of whose entries are zero. It also serves as the additive identity of the additive group of $m\times n$ matrices, and is denoted by the symbol $O$ or $0$ followed by subscripts corresponding to the dimension of the matrix as the context sees fit.^[1]^[2]^[3] Some examples of zero matrices are

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}}.\

Properties

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

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 $K_{m,n}\,$ .^[4] That is, for all $A\in K_{m,n}\,$ it satisfies the equation

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

There is exactly one zero matrix of any given dimension m×n (with entries from 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 is typically denoted by 0 without any subscript indicating the parent ring. Hence the examples above represent zero matrices over any ring.

The zero matrix also represents the linear transformation which sends all the vectors to the zero vector.^[5] It is idempotent, meaning that when it is multiplied by itself, the result is itself.

The zero matrix is the only matrix whose rank is 0.

Occurrences

In ordinary least squares regression, if there is a perfect fit to the data, the annihilator matrix is the zero matrix.

References

^ Lang, Serge (1987), Linear Algebra, Undergraduate Texts in Mathematics, Springer, p. 25, ISBN 9780387964126, We have a zero matrix in which a_ij = 0 for all i, j. ... We shall write it O.
^ "Intro to zero matrices (article) | Matrices". Khan Academy. Retrieved 2020-08-13.
^ Weisstein, Eric W. "Zero Matrix". mathworld.wolfram.com. Retrieved 2020-08-13.
^ Warner, Seth (1990), Modern Algebra, Courier Dover Publications, p. 291, ISBN 9780486663418, The neutral element for addition is called the zero matrix, for all of its entries are zero.
^ Bronson, Richard; Costa, Gabriel B. (2007), Linear Algebra: An Introduction, Academic Press, p. 377, ISBN 9780120887842, The zero matrix represents the zero transformation 0, having the property 0(v) = 0 for every vector v ∈ V.