How to describe line of equation instead of autonumbering

Question

Currently, I am using the \align method in this way:

\begin{align}
\mathcal{P}(Z \leq z) &= \mathcal{P}t\{f_1(\delta).f_2(\delta)\right\} \\
& = \exp(mt) \star \left\{\frac{l}{2\sqrt{\pi t^3}} \exp(-l^2/{4t})\right\} \\
& = F_1 * F_2
\end{align}

(sample code was found here)

This produces the following results (including an automatic equation line numbering system):

Would it be possible to replace those autonumbers with a description added in manually? I already know there is a way to remove the autonumbers, but I also would like to add some text to describe each line. Is this possible or too difficult to achieve?

Answer 1

You can use \tag*{<stuff>} to add descriptions without the surrounding (...) common to tags. If you want custom tags surrounded by (...), use \tag{<stuff>}.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
  \mathcal{P}(Z \leq z) &= \mathcal{P} t \{f_1(\delta).f_2(\delta) \} \tag{description1}                   \\
                        &= \exp(mt) \star \left\{ \frac{\ell}{2\sqrt{\pi t^3}} \exp(-\ell^2/{4t}) \right\} \\
                        &= F_1 \times F_2 \tag{description2}
\end{align*}

\end{document}

It's probably better to use a macro-like approach through something like

\newcommand{\eqdesc}[2][2em]{\tag*{#2}\hspace{#1}}

which inserts a 2em space at the end of the \tag*, effectively indenting it from the right-margin. This could separate it visually from other, regular \tags, if needed.

A more tabular-like display of descriptions is possible if you just add another equation into the mix:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
  \mathcal{P}(Z \leq z) &= \mathcal{P} t \{f_1(\delta).f_2(\delta) \} &
    \text{description1} \\
                        &= \exp(mt) \star \left\{ \frac{\ell}{2\sqrt{\pi t^3}} \exp(-\ell^2/{4t}) \right\} \\
                        &= F_1 \times F_2 &
    \text{description2}
\end{align*}

\end{document}

There will be a visible space between the equations to separate them.