General ⇒ Custom and automatic equation numbering

[cbustaam]

I all,
I'm trying to enumerate a group of "special" equations inside an align environment. I need something like the following

Code: Select all

.. = ..... (E1)
 .. = ....  (E2)
.. = ..... (E3)

but I want that this special numbering don's affect the ordinal equation numbering, it means, if the previous equation to the align environment is labeled as equation (4), the equation below this environment has to be labeled as (5).

I've trying a solution with the \tag command, but this command not is automatic (I have to write explicitly "E1", "E2", and "E3"). I only want to declare that the following equations number begin with "E" and next a number "automatically generated.

Any ideas?
Bests

[gmedina]

Hi,

try the environment myalign that I defined in the following example code:

Code: Select all

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcounter{oldequation}
\newenvironment{myalign}{%
  \setcounter{oldequation}{\theequation}
  \renewcommand\theequation{E\arabic{equation}}
  \setcounter{equation}{0}
  \start@align\@ne\st@rredfalse\m@ne
}{%
  \math@cr \black@\totwidth@
  \egroup
  \ifingather@
    \restorealignstate@
    \egroup
    \nonumber
    \ifnum0=`{\fi\iffalse}\fi
  \else
    $$%
  \fi
  \ignorespacesafterend
  \setcounter{equation}{\theoldequation}
  \renewcommand\theequation{\arabic{equation}}
}
\makeatother

\begin{document}

\begin{equation}
  a=b.
\end{equation}

\begin{myalign}
  a &= b\\
  &= c\\
  &= d
\end{myalign}

\begin{equation}
  a=b.
\end{equation}

\begin{equation}
  a=b.
\end{equation}

\begin{myalign}
  a &= b\\
  &= c\\
  &= d\\
  &= e\\
  &=f.
\end{myalign}

\begin{equation}
  a=b.
\end{equation}

\end{document} 

[cbustaam]

Thanks a lot gmedina.. it works great.
Best regards

[edmundfo]

gmedina, how would you change the align environment so that no equation numbers are displayed?

I ask about this because I am currently trying to modify the align environment so that everything in an align environment is displayed in a particular color. That is, I want a starred version of the following:

\newenvironment{balign}{%
\color{black}{%
}%
\align
\color{black}{%
}%
}{%
\color{black}{%
}%
\endalign
% extra material here
}

By the way, I believe that I should be able to achieve the same with \everydisplay{\color{black}}, but this only seems to affect the equation environment, and not the align environment.

[macnolds]

A simpler solution, using enumerate:

Code: Select all

\begin{enumerate}[(E1)]
\item $first$ \label{un}
\item $second$ \label{deux}
\item $third$. \label{trois}
\end{enumerate}

The only issue here is that when you try to reference one of these equations, it'll show a number such as "1" (or "(1)" with \eqref), instead of "E1."

[macnolds]

Here's a way to use the enumitem package in order to customize your iterated lists and get the "/ref" command to reference your equations properly.

Code: Select all

\usepackage{enumitem}
....
\begin{enumerate}[label=(E\arabic*), ref=(E\arabic*)]
\item blah \label{eq1}
\item blah \label{eq2}
\end{enumerate}

Check out equation \ref{eq1}.

[Stefan Kottwitz]

Hi macnolds,

welcome to the board!

You don't need to make the code bold. The forum provides a Code button for marking code, when editing. I did it for you above, so you can see how the result looks like.

Stefan

[macnolds]

Thanks, Stefan.