LATEX公式编号问题

给你个例子
\begin{gather}
x^2+y^2 = z^2 \label{eq:r2} \\
x^3+y^3 = z^3 \notag \\
x^4+y^4 = r^4 \tag{$*$} \\
x^5+y^5 = r^5 \tag*{$*$} \\
x^6+y^6 = r^6 \tag{\ref{eq:r2}$'$}
\end{gather}