テンソル積

$ A = \left( \begin{matrix} a_{00}&a_{01}\\a_{10}&a_{11} \end{matrix} \right)

$ B = \left( \begin{matrix} b_{00}&b_{01}\\b_{10}&b_{11} \end{matrix} \right)

$ A \otimes B = \left( \begin{matrix} a_{00}B&a_{01}B\\a_{10}B&a_{11}B \end{matrix} \right) = \left( \begin{matrix} a_{00}b_{00}&a_{00}b_{01}&a_{01}b_{00}&a_{01}b_{01}\\ a_{00}b_{10}&a_{00}b_{11}&a_{01}b_{10}&a_{01}b_{11}\\ a_{10}b_{00}&a_{10}b_{01}&a_{11}b_{00}&a_{11}b_{01}\\ a_{10}b_{10}&a_{10}b_{11}&a_{11}b_{10}&a_{11}b_{11}\\ \end{matrix} \right)