二項積
I × ×  A = ( A × ×  I ) I − A ⊤ , {\displaystyle \mathbf {I} \mathop {_{\textstyle \times }} \limits ^{\textstyle \times }\mathbf {A} =(\mathbf {A} \mathop {_{\textstyle \times }} \limits ^{\textstyle \times }\mathbf {I} )\mathbf {I} -\mathbf {A} ^{\top },}
I : a b = ( I ⋅ a ) ⋅ b = a ⋅ b = tr  ( a b ) . {\displaystyle \mathbf {I} :\mathbf {ab} =(\mathbf {I} \cdot \mathbf {a} )\cdot \mathbf {b} =\mathbf {a} \cdot \mathbf {b} =\operatorname {tr} (\mathbf {ab} ).}
ここで、 "tr" は対角和(蹟、trace)を表す。