So, I've read the discussions here and also here but neither seem to solve my problem. I'd like to break a long list of matrices between pages, problem is I'm using an alignedat (and sometimes a gathered) environment inside an equation one, so it is unbreakable, i.e. using \allowdisplaybreaks or \displaybreak doesn't work.
There are a couple of problems in using an align or align* environment instead: the first one is that I want a single tag for the overall matrices, this could be resolved with manually tagging inside align* but it's incredibly tedious because I have the same problem all over my thesis, moreover it's hard to control the position of the tags to keep them centered and they are not automatically realigned if I change something, I'd love a global solution. Another problem is that the matrices I'm writing are 5x5 and for reading simplicity is useful to keep some columns/rows blank when they're not needed. This seems to work fine inside an equation+alignedat environment but not having values in the first column completly breaks the align environment.
The best would be to have a somewhat global (or better yet, something that could be applied to just some chapters) solution. As for the tag I think that the best rendering would be to have the same number tag centered in on the piece that stays at the bottom of the page and in the one tha goes to the new page.
Here an example of what I'd want graphically, just to get an idea. The breaking is automatically managed by latex but the tag isn't... If I modify something and the matrices break on another line I have to manually reposition the tags
\documentclass[a4paper, 11pt, twoside]{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage{amsmath, amssymb}
\begin{document}
some long text
\vspace{10cm}
\begingroup
\allowdisplaybreaks
\begin{align*}
C'_{11}-C'_{22}&=\begin{pmatrix}
1 & 0 & 0 & 0 & 0\\
0 & -1 & 0 & 0 & 0\\
0 & 0 & 0 & 0 & 0\\
0 & 0 & 0 & 0 & 0\\
0 & 0 & 0 & 0 & 0
\end{pmatrix},\\[10pt]
\sqrt{\frac{5}{3}}(C'_{11}+C'_{22})&=\frac{1}{\sqrt{15}}\begin{pmatrix}
3 & 0 & 0 & 0 & 0\\
0 & 3 & 0 & 0 & 0\\
0 & 0 & -2 & 0 & 0\\
0 & 0 & 0 & -2 & 0\\
0 & 0 & 0 & 0 & -2
\end{pmatrix},\stepcounter{equation}\tag{\theequation} \label{Eq:Gen_diag_SU(5)}\\[10pt]
C'_{33}-C'_{44}&=\begin{pmatrix}
0 & 0 & 0 & 0 & 0\\
0 & 0 & 0 & 0 & 0\\
0 & 0 & 1 & 0 & 0\\
0 & 0 & 0 & -1 & 0\\
0 & 0 & 0 & 0 & 0
\end{pmatrix},\\[10pt]
\sqrt{\frac{5}{3}}(C'_{33}+C'_{44})&=\frac{1}{\sqrt{15}}\begin{pmatrix}
-2 & 0 & 0 & 0 & 0\\
0 & -2 & 0 & 0 & 0\\
0 & 0 & 3 & 0 & 0\\
0 & 0 & 0 & 3 & 0\\
0 & 0 & 0 & 0 & -2
\end{pmatrix}\tag{\theequation}
\end{align*}
\endgroup
\end{document}
Here instead the matrices and environments I'm working with
\documentclass[a4paper, 11pt, twoside]{report}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage{amsmath, amssymb}
\begin{document}
\begin{equation}
\begin{alignedat}{2}
\Tilde{\lambda}_9&=\begin{pmatrix}
& & & 1 & 0\\
& & & 0 & 0\\
& & & 0 & 0\\
1 & 0 & 0 & & \\
0 & 0 & 0 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{10}&=\begin{pmatrix}
& & & -i & 0\\
& & & 0 & 0\\
& & & 0 & 0\\
i & 0 & 0 & & \\
0 & 0 & 0 & &
\end{pmatrix}\\[10pt]
%
\Tilde{\lambda}_{11}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 1 & 0\\
& & & 0 & 0\\
0 & 1 & 0 & & \\
0 & 0 & 0 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{12}&=\begin{pmatrix}
& & & 0 & 0\\
& & & -i & 0\\
& & & 0 & 0\\
0 & i & 0 & & \\
0 & 0 & 0 & &
\end{pmatrix}\\[10pt]
%
\Tilde{\lambda}_{13}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & 0\\
& & & 1 & 0\\
0 & 0 & 1 & & \\
0 & 0 & 0 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{14}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & 0\\
& & & -i & 0\\
0 & 0 & i & & \\
0 & 0 & 0 & &
\end{pmatrix}\\[10pt]
%
\Tilde{\lambda}_{15}&=\begin{pmatrix}
& & & 0 & 1\\
& & & 0 & 0\\
& & & 0 & 0\\
0 & 0 & 0 & & \\
1 & 0 & 0 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{16}&=\begin{pmatrix}
& & & 0 & -i\\
& & & 0 & 0\\
& & & 0 & 0\\
0 & 0 & 0 & & \\
i & 0 & 0 & &
\end{pmatrix}\\[10pt]
%
\Tilde{\lambda}_{17}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & 1\\
& & & 0 & 0\\
0 & 0 & 0 & & \\
0 & 1 & 0 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{18}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & -i\\
& & & 0 & 0\\
0 & 0 & 0 & & \\
0 & i & 0 & &
\end{pmatrix}\\[10pt]
%
\Tilde{\lambda}_{19}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & 0\\
& & & 0 & 1\\
0 & 0 & 0 & & \\
0 & 0 & 1 & &
\end{pmatrix} &\qquad\qquad
\Tilde{\lambda}_{20}&=\begin{pmatrix}
& & & 0 & 0\\
& & & 0 & 0\\
& & & 0 & -i\\
0 & 0 & 0 & & \\
0 & 0 & i & &
\end{pmatrix}
\end{alignedat}
\end{equation}
\end{document}
