A test of Equation Numbering

Equation: $$E = mc^2$$ Equation*: \begin{equation*}$$E = mc^2$$\end{equation*}
Brackets: $E = mc^2$ Brackets tagged: $E = mc^2\tag{x}$
Split: $$\begin{split} a& =b+c-d\\ & \quad +e-f\\ & =g+h\\ & =i \end{split}$$
Multline: \begin{multline} a+b+c+d+e+f+g\\ M+N+O+P+Q\\ R+S+T\\ u+v+w+x+y+z \end{multline} Multline*: \begin{multline*} a+b+c+d+e+f+g\\ M+N+O+P+Q\\ R+S+T\\ u+v+w+x+y+z \end{multline*}
Gather: \begin{gather} a_1=b_1+c_1\\ a_2=b_2+c_2-d_2+e_2 \end{gather} Gather*: \begin{gather*} a_1=b_1+c_1\\ a_2=b_2+c_2-d_2+e_2 \end{gather*}
Align: \begin{align} a_1& =b_1+c_1\\ a_2& =b_2+c_2-d_2+e_2 \end{align} Align*: \begin{align*} a_1& =b_1+c_1\\ a_2& =b_2+c_2-d_2+e_2 \end{align*} Align: \begin{align} a_{11}& =b_{11}& a_{12}& =b_{12}\\ a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} \end{align} Align with \notag and \tag: \begin{align} a_{11}& =b_{11}& a_{12}& =b_{12}\notag\\ a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} \tag{y} \end{align} Align* with \tag: \begin{align*} a_1& =b_1+c_1\tag{z}\\ a_2& =b_2+c_2-d_2+e_2 \end{align*}