In mathematics, a polynomial matrix or matrix of polynomials is a matrix whose elements are univariate or multivariate polynomials. Equivalently, a polynomial matrix is a polynomial whose coefficients are matrices.

A univariate polynomial matrix P of degree p is defined as:

${\displaystyle P=\sum _{n=0}^{p}A(n)x^{n}=A(0)+A(1)x+A(2)x^{2}+\cdots +A(p)x^{p))$

where ${\displaystyle A(i)}$ denotes a matrix of constant coefficients, and ${\displaystyle A(p)}$ is non-zero. An example 3×3 polynomial matrix, degree 2:

${\displaystyle P={\begin{pmatrix}1&x^{2}&x\\0&2x&2\\3x+2&x^{2}-1&0\end{pmatrix))={\begin{pmatrix}1&0&0\\0&0&2\\2&-1&0\end{pmatrix))+{\begin{pmatrix}0&0&1\\0&2&0\\3&0&0\end{pmatrix))x+{\begin{pmatrix}0&1&0\\0&0&0\\0&1&0\end{pmatrix))x^{2}.}$

We can express this by saying that for a ring R, the rings ${\displaystyle M_{n}(R[X])}$ and ${\displaystyle (M_{n}(R))[X]}$ are isomorphic.

Properties

• A polynomial matrix over a field with determinant equal to a non-zero element of that field is called unimodular, and has an inverse that is also a polynomial matrix. Note that the only scalar unimodular polynomials are polynomials of degree 0 – nonzero constants, because an inverse of an arbitrary polynomial of higher degree is a rational function.
• The roots of a polynomial matrix over the complex numbers are the points in the complex plane where the matrix loses rank.
• The determinant of a matrix polynomial with Hermitian positive-definite (semidefinite) coefficients is a polynomial with positive (nonnegative) coefficients.^[1]

Note that polynomial matrices are not to be confused with monomial matrices, which are simply matrices with exactly one non-zero entry in each row and column.

If by λ we denote any element of the field over which we constructed the matrix, by I the identity matrix, and we let A be a polynomial matrix, then the matrix λI − A is the characteristic matrix of the matrix A. Its determinant, |λI − A| is the characteristic polynomial of the matrix A.