Kronecker Product of Matrices
Definition1
The Kronecker product of two matrices $A = [a_{ij}] \in M_{m \times n}$, $B \in M_{p \times q}$ is defined as follows.
$$ A \otimes B := \begin{bmatrix} a_{11} B & \cdots & a_{1n} B \\ \vdots & \ddots & \vdots \\ a_{m1} B & \cdots & a_{mn} B \end{bmatrix} \in M_{mp \times nq} $$
Explanation
The matrix representation of the tensor product of two linear transformations is defined by the Kronecker product of each matrix representation of the linear transformations.
Unlike multiplication, it is well-defined regardless of the size of the matrices.
Properties
For a matrix $A, B, C, D$ and a scalar $k$, the following hold:
- Scalar multiplication: $(kA) \otimes B = k (A \otimes B) = A \otimes (kB) \\[0.5em]$
- Matrix multiplication: $(A \otimes B) (C \otimes D) = (AC) \otimes (BD) \\[0.5em]$
- Associative law: $(A \otimes B) \otimes C = A \otimes (B \otimes C) \\[0.5em]$
- Distributive law: $A \otimes ( B + C ) = A \otimes B + A \otimes C \\[0.5em]$
- Transpose: $(A \otimes B)^{T} = A^{T} \otimes B^{T} \\[0.5em]$
- Trace: $\tr(A \otimes B) = \tr(A) \tr(B) \\[0.5em]$
- Inverse: $(A \otimes B)^{-1} = A^{-1} \otimes B^{-1}$
- if $A \otimes B$ is invertible, then $\iff$ $A$ and $B$ are invertible, then $\\[0.5em]$
- Determinant: $\det (A \otimes B) = \det(A)^{n} \det(B)^{p},\quad A\in M_{n \times n}, B \in B_{p \times p} \\[0.5em]$
Like matrix multiplication, the commutative law does not hold. Simply look at the multiplication of $A = \begin{bmatrix} a & b \\ c & d \end{bmatrix}$ and the identity matrix $I_{2}$,
$$ A \otimes I_{2} = \begin{bmatrix} a & 0 & b & 0 \\ 0 & a & 0 & b \\ c & 0 & d & 0 \\ 0 & c & 0 & d \end{bmatrix} \ne \begin{bmatrix} a & b & 0 & 0 \\ c & d & 0 & 0 \\ 0 & 0 & a & b \\ 0 & 0 & c & d \end{bmatrix} = I_{2} \otimes A $$
김영훈·허재성, 양자 정보 이론 (2020), p37 ↩︎