# How to number a given equation in the align* environment such that the number depends on the whole document?

\documentclass{amsart}

\begin{document}
We have
\begin{align*}
x^{2} &= x\cdot x\\
&= x^{6/3}
\end{align*}
for all real $x$
\end{document}


To illustrate, suppose I would like to number only the first equality. I tried the command \tag{a} where $a$ is a natural number, but the number thus generated seems to be fixed, i.e. I would still get the label (a) even if there is another labeled equation preceding equation (a). What I want is that (a) should be (a+1) when there is another equation labeled before equation (a), say.

While it is possible to tag equations in an align* environment, the \tag command does indeed uses a fixed equation 'number' etc. then.

The align* environment however does not count the equation number, but the align environment does. If specific equations should not be numbered, \notag suppresses the numbering then.

\documentclass{amsart}

\begin{document}
We have
\begin{align}
x^{2} &= x\cdot x \\
&= x^{6/3} \notag
\end{align}

And once again:

\begin{align}
x^{2} &= x\cdot x  \\
&= x^{6/3} \notag
\end{align}

for all real $x$
\end{document}


However ... if align* is used, a non-fixed tagging can be used with a faking command, say \tagitanyway.

This command has an optional argument, which defaults to equation and must be a counter name. This counter is \refstepcountered then and a 'fixed' tag is applied, using \the'countername'.

\documentclass{amsart}

\newcommand{\tagitanyway}[1][equation]{\refstepcounter{#1}\tag{\csname the#1\endcsname}}

\begin{document}

$$E = mc^2$$
We have

\begin{align*}
x^{2} &= x\cdot x \tagitanyway \\
&= x^{6/3}
\end{align*}

And once again:

\begin{align*}
x^{2} &= x\cdot x \tagitanyway \\
& = x^{6/3} \\
& = x^{42/21} \tagitanyway
\end{align*}

for all real $x$
\end{document}


• Thanks. I did not make my question clear enough. Sorry that I have to say that I literally know the align trick with \nonumber or \notag. But, since at most of time I need occasionally to number equations, I think align* solution would be better; for otherwise I have to extensively type "\nonumber" or "\notag". I hope now I have made it clear. :)
– Yes
Jul 7, 2015 at 2:32
• @Chou: Yes, this clearification would helped me ;-)
– user31729
Jul 7, 2015 at 2:33
• Thank you. It is somewhat disappointed to know that there is no such "service" out there for align*, frankly.
– Yes
Jul 7, 2015 at 2:36
• @Chou: I believe there is but I don't know/remember -- see my updated 'example' -- perhaps, I've reinvented the wheel ;-)
– user31729
Jul 7, 2015 at 2:41
• @Sigur: Well, I thought of subequation, for example?
– user31729
Jul 7, 2015 at 10:19