Sistema in cui ogni equazione e’ numerata

Ho postato lo stesso probelma sul NG francese ed ecco la risposta (a naso doveva essere facile ma proprio non ne uscivo da sola):

pacchetto cases con i suoi environnement numcases e subnumcases

Dal file .sty:

% C A S E S . S T Y ver 2.5 May 2002
%
% Copyright (C) 1993,1994,1995,2000,2002 by Donald Arseneau
% asnd@triumf.ca
% These macros may be freely transmitted, reproduced, or modified
% provided that this notice is left intact. Sub-equation numbering
% is based on subeqn.sty by Stephen Gildea; most of the rest is based
% on LaTeX’s \eqnarray by Leslie Lamport and the LaTeX3 team.
%
% This provides a LaTeX environment {numcases} to produce multi-case
% equations with a separate equation number for each case. There is
% also {subnumcases} which numbers each case with the overall equation
% number plus a letter [8a, 8b, etc.]. The syntax is
%
% \begin{numcases}{left_side}
% case_1 & explanation_1 \\
% case_2 & explanation_2 \\
% …
% case_n & explanation_n
% \end{numcases}
%
% Each case is a math formula, and each explanation is a piece of lr mode
% text (which may contain math mode in \(…\) or $…$). The explanations
% are optional. Equation numbers are inserted automatically, just as for
% the eqnarray environment. In particular, the \nonumber command suppresses
% an equation number and the \label command allows reference to a particular
% case. In a subnumcases environment, a \label in the left_side of the
% equation gives the overall equation number, without any letter.
%
% To use this package,
% include “\usepackage{cases}” after \documentclass. You may also
% specify “\usepackage[subnum]{cases}” to force *all* numcases
% environments to be treated as subnumcases.
%
% Question: Is there a {numcases*} environment for unnumbered cases?
% Answer: There is a {cases} environment in AMS-LaTeX, but it is just as
% convenient to stick with the canonical LaTeX array:
% \[ left side = \left\{ PROTECTED1 \right. \]
%
% Speaking of AMS-math, they use an entirely different system of equation
% numbering, and this package uses ordinary LaTeX numbering.
%
% – – – – –
% A simple example is:
% \begin{numcases}{|x|=}
% x, & for $x \geq 0$\\
% -x, & for $x < 0$ % \end{numcases} % % Giving: % / x for x > 0 (1)
% |x| = < - % \ -x for x < 0 (2) % % - - - - - % % Another example is calculating the square root of $c+id$. First compute % \begin{subnumcases}{\label{w} w\equiv} % 0 & $c = d = 0$\label{wzero}\\ % \sqrt{|c|}\,\sqrt{\frac{1 + \sqrt{1+(d/c)^2}}{2}} & $|c| \geq |d|$ \\ % \sqrt{|d|}\,\sqrt{\frac{|c/d| + \sqrt{1+(c/d)^2}}{2}} & $|c| < |d|$ % \end{subnumcases} % Then, using $w$ from eq.~(\ref{w}), the square root is % \begin{subnumcases}{\sqrt{c+id}=} % 0 & $w=0$ (case \ref{wzero})\\ % w+i\frac{d}{2w} & $w \neq 0$, $c \geq 0$ \\ % \frac{|d|}{2w} + iw & $w \neq 0$, $c < 0$, $d \geq 0$ \\ % \frac{|d|}{2w} - iw & $w \neq 0$, $c < 0$, $d < 0$ % \end{subnumcases}