1

To label a single symbol to long equation, we use \begin{split}...\end{split}:

   \begin{align}\label{eq1}
   \begin{split}
   H=\langle x_1,x_2,\cdots, x_n \colon & [x_i,x_j,x_k]=1,\\
                                              & [x_i,x_j]^p=1 \rangle.
   \end{split}
   \end{align}

To label an equation by a symbol say (*), we use \tag{ } inside \begin{equation}.

\begin{equation}
E=mc^2\tag{*}
\end{equation}

Question: If I want to label a long equation by a single symbol, how to do it?

2

you have very nearly answered your own question.

\begin{equation} ... \end{equation} should be used for display math that consists of a single formula, i.e., has only one equation "number". split can be used within equation, and is recommended (read the documentation, please). so

\begin{equation}\label{eq1}
\begin{split}
H=\langle x_1,x_2,\cdots, x_n \colon & [x_i,x_j,x_k]=1,\\
                                          & [x_i,x_j]^p=1 \rangle.
\end{split}
\tag{*}
\end{equation}

will do exactly what you want.

  • Or aligned. But the main request is to also use something like \tag{<symbol>}. – Werner Mar 2 '16 at 2:24
  • sorry, I didn't get your your answer. where we use \tag here? – p Groups Mar 2 '16 at 2:25
  • @pGroups -- sorry, not paying proper attention. fixed now. – barbara beeton Mar 2 '16 at 2:30
  • @barbara: Thanks. (Actually, I was checking this by adding \tag{ } at different places inside \begin{equation...\end{equation}, but it was giving errors. Then I thought that there may be some different way for it, hence posted the question above.) – p Groups Mar 2 '16 at 3:10

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