# Kronecker Product¶

class fastmat.Kron

Bases: fastmat.Matrix.Matrix

For matrices $$A_i \in \mathbb{C}^{n_i \times n_i}$$ for $$i = 1,\dots,k$$ the Kronecker product

$A_1 \otimes A_2 \otimes \dots \otimes A_k$

can be defined recursively because of associativity from the Kronecker product of $$A \in \mathbb{C}^{n \times m}$$ and $$B \in \mathbb{C}^{r \times s}$$ defined as

$\begin{split}A \otimes B = \begin{bmatrix} a_{11} B & \dots & a_{1m} B \\ \vdots & \ddots & \vdots \\ a_{n1} B & \dots & a_{nm} B \end{bmatrix}.\end{split}$

We make use of a decomposition into a standard matrix product to speed up the matrix-vector multiplication which is introduced in . This then yields multiple benefits:

• It already brings down the complexity of the forward and backward projection if the factors $$A_i$$ have no fast transformations.
• It is not necessary to compute the matrix representation of the product, which saves a lot of memory.
• When fast transforms of the factors are available the calculations can be sped up further.
>>> # import the package
>>> import fastmat as fm
>>>
>>> # define the factors
>>> C = fm.Circulant(x_C)
>>>
>>> # define the Kronecker
>>> # product
>>> P = fm.Kron(C.H, H)


Assume we have a circulant matrix $$C$$ with first column $$x_c$$ and a Hadamard matrix $${\mathcal{H}}_n$$ of order $$n$$. Then we define

$P = C^\mathrm{H} \otimes H_n.$
__init__

Initialize a Kron matrix instance.

Parameters: *matrices : fastmat.Matrix The matrix instances to form a kronecker product of. Currently only square matrices are supported as kronecker product terms. **options : optional Additional keyworded arguments. Supports all optional arguments supported by fastmat.Matrix.