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 A {\displaystyle A} of degree n {\displaystyle n} is defined as:
A ( x ) = ∑ i = 0 n A i x i = A 0 + A 1 x + A 2 x 2 + ⋯ + A n x n , {\displaystyle A(x)=\sum _{i=0}^{n}A_{i}x^{i}=A_{0}+A_{1}x+A_{2}x^{2}+\cdots +A_{n}x^{n},}
where A i {\displaystyle A_{i}} denotes a matrix of constant coefficients, and A n {\displaystyle A_{n}} is non-zero. An example 3×3 polynomial matrix, degree 2:
A ( x ) = ( 1 x 2 x 0 2 x 2 3 x + 2 x 2 − 1 0 ) = ( 1 0 0 0 0 2 2 − 1 0 ) + ( 0 0 1 0 2 0 3 0 0 ) x + ( 0 1 0 0 0 0 0 1 0 ) x 2 . {\displaystyle A(x)={\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 M n ( R [ X ] ) {\displaystyle M_{n}(R[X])} and
( M n ( R ) ) [ X ] {\displaystyle (M_{n}(R))[X]} are isomorphic.
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