Ciao Enrio, non sono d’accordo. A volte si ha bisogno di fare riferimento al sistema intero, altre volte alla singola equazione. Penso ad esempio al sistema dui Bear-Nunziato composto da 7 PDE; per semplificarlo si eliminano alcune PDE con delle chiusure algebriche ed è ben piu comodo dire “l’eq 2.14c” piuttosto che la terza equazione del sistema “2.14” ecc…
Comunque, ritornando alla richiesta iniziale, LaTeX ha già questa funzione ! È fornita dal pacchetto “cases” :
`% 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:
% \[ left side = \left\{ \begin{array}…\end{array} \right. \]
%
% 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}
%`