# Define a new align environment with custom numeration

I am writting a report and I have multiple systems of equations. I want to avoid have too big numbers (Equation 4.200 for example) and I dont want to link the numbering to the chapter or the section.

I want a personalized align enviroment that enummerates the equations as:

(A1)

(A2)

(A3)

The first time I use it and

(B1)

(B2)

(B3)

The second time I use and so on.

The normal numeration of the usual align enviroment or equation enviroment should not be afffected.

I have been trying for some hours but none of the answers seems to work :(

We can locally change the equation counter (restoring it at the end). This also behaves well with \label and \ref.

\documentclass{article}
\usepackage{amsmath}

\newcounter{alphalign}
\renewcommand{\thealphalign}{\Alph{alphalign}}
\newcounter{alphalignsavedequation}

\newenvironment{alphalign}
{%
\setcounter{alphalignsavedequation}{\value{equation}}%
\counterwithin*{equation}{alphalign}%
\stepcounter{alphalign}%
\renewcommand{\theequation}{\thealphalign\arabic{equation}}%
\align
}
{\endalign\setcounter{equation}{\value{alphalignsavedequation}}}

\begin{document}

An equation
$$0=0$$
An alignment with alpha
\begin{alphalign}
0&=0\\
1&=1\\
2&=2
\end{alphalign}
Another equation
$$0=0$$
Another alignment with alpha
\begin{alphalign}
0&=0\\
1&=1\\
2&=2
\end{alphalign}
Another equation
$$0=0$$

\end{document}


The following code \lets the equation counter to another counter called selfalign within the newly-defined environment (group) selfalign. Since the \let is scoped/temporary within the group, it reverts to the original definition afterwards. alphalph is used to ensure the selfalign counter prefix can roll over beyond Z, if needed.

\documentclass{article}

\usepackage{amsmath,alphalph}
\newcounter{selfalignprefix}
\newcounter{selfalign}[selfalignprefix]

\renewcommand{\theselfalignprefix}{\AlphAlph{\value{selfalignprefix}}}
\renewcommand{\theselfalign}{\theselfalignprefix\arabic{selfalign}}

\makeatletter
\newenvironment{selfalign}
{\let\c@equation\c@selfalign% Make equation counter point to the selfalign counter
\stepcounter{selfalignprefix}% Step the prefix counter
\renewcommand{\theequation}{\theselfalign}% Update equation counter representation
\align}
{\endalign}
\makeatother

\begin{document}

\begin{align}
f(x) \\ g(x) \\ h(x)
\end{align}

\begin{selfalign}
f(x) \\ g(x) \\ h(x)
\end{selfalign}

\begin{align}
f(x) \\ g(x) \\ h(x)
\end{align}

\begin{selfalign}
f(x) \\ g(x) \\ h(x)
\end{selfalign}

\end{document}