张量积(tensor product)又叫克罗内克积(Kroneciker product)，两个列向量$a$和$b$的张量积$\mathbf{a}\otimes \mathbf{b}$定义为：

a ⊗ b = [ a 1 a 2 ⋮ a m ] m × 1 ⊗ [ b 1 b 2 ⋮ b n ] n × 1 = a b T = [ a 1 b 1 a 1 b 2 ⋯ a 1 b n a 2 b 1 a 2 b 2 ⋯ a 2 b n ⋮ ⋮ ⋱ ⋮ a m b 1 a m b 2 ⋯ a m b n ] \mathbf{a} \otimes \mathbf{b}=\begin{bmatrix} a_1\\ a_2\\ \vdots\\ a_m \end{bmatrix}_{m\times1}\otimes \begin{bmatrix} b_1\\ b_2\\ \vdots\\ b_n \end{bmatrix}_{n\times1}=\mathbf{a}\mathbf{b}^T=\begin{bmatrix} a_1 b_1 & a_1 b_2 & \cdots & a_1 b_n \\ a_2 b_1 & a_2 b_2 & \cdots & a_2 b_n \\ \vdots & \vdots & \ddots & \vdots \\ a_m b_1 & a_m b_2 & \cdots & a_m b_n \end{bmatrix} a⊗b= ​a1​a2​⋮am​​ ​m×1​⊗ ​b1​b2​⋮bn​​