# Use equation and cases for big and numerous equation

I have a system of differential equations I want to write. Some equations are very long and there are numerous.

I wonder if there is a way to split the equations (automaticaly if possible) and to split the system. And any advide on how handle big systems is welcomed. (write the system is a good/bad idea, a table is better... anything you want)

Here is a prototype of my code

\documentclass[a4paper]{article}
\usepackage{amssymb}
\usepackage{amsmath}

\begin{document}
$$\begin{cases} \frac{dx}{dt} = \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma\\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \frac{dx}{dt} = \nu\alpha\beta\gamma \\ \end{cases}$$
\end{document}


Some suggestions/observations:

• Since the system of differential equations looks like it might span more than 1 page, you should not use an equation environment. Instead, use either an align or align* (the latter if you do not need equation numbers) environment, along with an \allowdisplaybreaks statement.

• It seems pointless to use a cases environment if the equations end up spanning 2 or more pages. However, I may be missing something.

• If you have a long expression on the right-hand side, encase it in a \parbox directive, as is shown below. That way, you'll get automatic line-breaking, which would appear to be one of your objectives.

\documentclass{article}
\usepackage{amssymb,amsmath}
\allowdisplaybreaks
\begin{document}

\begin{align*}
\frac{dx}{dt} &= \parbox[t]{0.75\textwidth}{\raggedright%
$\nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma + \nu\alpha\beta\gamma$}\\
\frac{dx}{dt} &= \nu\alpha\beta\gamma \\
\frac{dx}{dt} &= \nu\alpha\beta\gamma \\
\frac{dx}{dt} &= \nu\alpha\beta\gamma
\end{align*}

\end{document}

• * align or align*with \allowdisplaybreaks instead of equation ok * I don't understand the cases part * what do the parameters of parboxmean ? * With this version, we loose the bracket... – Ccile Oct 25 '17 at 19:44
• @Mico: Why a \parbox for which you have to specify a width, whereas aligned or multlined don't require it? – Bernard Oct 25 '17 at 19:45
• @Bernard - the OP asked for automatic linebreaking. Inline-style math in a parbox will deliver just that, whereas the aligned and multlined environments will not. The only, and fairly minor, cost of using a \parbox is that a width has to be selected. 0.75\textwidth is obviously merely an example, but it happens to work in the present case. :-) – Mico Oct 25 '17 at 20:01
• @Ccile - What is a curly brace that spans more than one page even supposed to signify? I honestly have no idea. About the \parbox ("paragraph box") directive: [t] means that top-alignment is requested (the default is centering), and 0.75\texwidth` specifies the desired width. – Mico Oct 25 '17 at 20:03
• Curly brace on 2 pages -> I have no idea too, it was just in case of... Ok, thanks – Ccile Oct 25 '17 at 20:15