 Diagonal matrix

In linear algebra, a diagonal matrix is a matrix (usually a square matrix) in which the entries outside the main diagonal (↘) are all zero. The diagonal entries themselves may or may not be zero. Thus, the matrix D = (d_{i,j}) with n columns and n rows is diagonal if:
For example, the following matrix is diagonal:
The term diagonal matrix may sometimes refer to a rectangular diagonal matrix, which is an mbyn matrix with only the entries of the form d_{i,i} possibly nonzero. For example:
 or
However, in the remainder of this article we will consider only square matrices. Any square diagonal matrix is also a symmetric matrix. Also, if the entries come from the field R or C, then it is a normal matrix as well. Equivalently, we can define a diagonal matrix as a matrix that is both upper and lowertriangular. The identity matrix I_{n} and any square zero matrix are diagonal. A onedimensional matrix is always diagonal.
Contents
Scalar matrix
A diagonal matrix with all its main diagonal entries equal is a scalar matrix, that is, a scalar multiple λI of the identity matrix I. Its effect on a vector is scalar multiplication by λ. For example, a 3×3 scalar matrix has the form:
The scalar matrices are the center of the algebra of matrices: that is, they are precisely the matrices that commute with all other square matrices of the same size.
For an abstract vector space V (rather than the concrete vector space K^{n}), or more generally a module M over a ring R, with the endomorphism algebra End(M) (algebra of linear operators on M) replacing the algebra of matrices, the analog of scalar matrices are scalar transformations. Formally, scalar multiplication is a linear map, inducing a map (send a scalar λ to the corresponding scalar transformation, multiplication by λ) exhibiting End(M) as a Ralgebra. For vector spaces, or more generally free modules , for which the endomorphism algebra is isomorphic to a matrix algebra, the scalar transforms are exactly the center of the endomorphism algebra, and similarly invertible transforms are the center of the general linear group GL(V), where they are denoted by Z(V), follow the usual notation for the center.
Matrix operations
The operations of matrix addition and matrix multiplication are especially simple for diagonal matrices. Write diag(a_{1},...,a_{n}) for a diagonal matrix whose diagonal entries starting in the upper left corner are a_{1},...,a_{n}. Then, for addition, we have
 diag(a_{1},...,a_{n}) + diag(b_{1},...,b_{n}) = diag(a_{1}+b_{1},...,a_{n}+b_{n})
and for matrix multiplication,
 diag(a_{1},...,a_{n}) · diag(b_{1},...,b_{n}) = diag(a_{1}b_{1},...,a_{n}b_{n}).
The diagonal matrix diag(a_{1},...,a_{n}) is invertible if and only if the entries a_{1},...,a_{n} are all nonzero. In this case, we have
 diag(a_{1},...,a_{n})^{1} = diag(a_{1}^{1},...,a_{n}^{1}).
In particular, the diagonal matrices form a subring of the ring of all nbyn matrices.
Multiplying an nbyn matrix A from the left with diag(a_{1},...,a_{n}) amounts to multiplying the ith row of A by a_{i} for all i; multiplying the matrix A from the right with diag(a_{1},...,a_{n}) amounts to multiplying the ith column of A by a_{i} for all i.
Other properties
The eigenvalues of diag(a_{1}, ..., a_{n}) are a_{1}, ..., a_{n} with associated eigenvectors of e_{1}, ..., e_{n}, where the vector e_{i} is all zeros except a one in the ith row. The determinant of diag(a_{1}, ..., a_{n}) is the product a_{1}...a_{n}.
The adjugate of a diagonal matrix is again diagonal.
A square matrix is diagonal if and only if it is triangular and normal.
Uses
Diagonal matrices occur in many areas of linear algebra. Because of the simple description of the matrix operation and eigenvalues/eigenvectors given above, it is always desirable to represent a given matrix or linear map by a diagonal matrix.
In fact, a given nbyn matrix A is similar to a diagonal matrix (meaning that there is a matrix X such that X^{1}AX is diagonal) if and only if it has n linearly independent eigenvectors. Such matrices are said to be diagonalizable.
Over the field of real or complex numbers, more is true. The spectral theorem says that every normal matrix is unitarily similar to a diagonal matrix (if AA^{*} = A^{*}A then there exists a unitary matrix U such that UAU^{*} is diagonal). Furthermore, the singular value decomposition implies that for any matrix A, there exist unitary matrices U and V such that UAV^{*} is diagonal with positive entries.
Operator theory
In operator theory, particularly the study of PDEs, operators are particularly easy to understand, and PDEs easy to solve, if the operator is diagonal with respect to the basis one is working with – this corresponds to a separable partial differential equation. Thus, a key technique to understand operators is to have a change of coordinates – in the language of operators, an integral transform – which changes the basis to an eigenbasis of eigenfunctions: which makes the equation separable. An important example of this is the Fourier transform, which diagonalizes constant coefficient differentiation operators (or more generally translation invariant operators), such as the Laplacian operator, say, in the heat equation.
Especially easy are multiplication operators, which are defined as multiplication by (the values of) a fixed function – the values of the function at each point correspond to the diagonal entries of a matrix.
See also
 Antidiagonal matrix
 Banded matrix
 Bidiagonal matrix
 Diagonally dominant matrix
 Diagonalizable matrix
 Multiplication operator
 Tridiagonal matrix
 Toeplitz matrix
 Toral Lie algebra
 Circulant matrix
References
 Roger A. Horn and Charles R. Johnson, Matrix Analysis, Cambridge University Press, 1985. ISBN 0521305861 (hardback), ISBN 0521386322 (paperback).
Categories: Matrix normal forms
 Sparse matrices
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