Multiline equation and text

Question

I would really like to get a multiline equation with text explaining each step.

I've got close with

\begin{equation*}
\begin{split}
    A \simeq & B  \mbox{[Lemma 8.35(iii)]} \\
      \simeq & C \mbox{[exactness of first column - see Ex. 5.3]} \\
      etc
\end{split}
\end{equation*} `

But I can't get tab spacing between the equation and the message box. Maybe an mbox is not the best way?

Answer 1

The amsmath package offers a number of environments that can achieve what you want: here's an example (with annotations flushed right, and with annotations two quads appart from the equations):

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
    A & \simeq B  \tag{[Lemma 8.35(iii)]} \\
        & \simeq C  \tag{[exactness of first column - see Ex. 5.3]}
\end{align*} `

\begin{alignat*}{2}
  A &\simeq B   &\qquad & \text{[Lemma 8.35(iii)]} \\
      &\simeq C   &\qquad & \text{[exactness of first column - see Ex. 5.3]}
\end{alignat*} `

\end{document}

Answer 2

You can also use intertext for short annotations of 1-2 lines.

\begin{align}
A_1&=N_0(\lambda;\Omega’)-\phi(\lambda;\Omega’),\\
A_2&=\phi(\lambda;\Omega’)-\phi(\lambda;\Omega),\\
\intertext{and}
A_3&=\mathcal{N}(\lambda;\omega).
\end{align}

see 3.10 Interrupting a display in amsmath