How to align an equation using amsmath

Question

Can somebody explain to me how to put this in proper LaTeX-Code?

I am really not sure... let's see:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
\sum_{(i,j)\el E} x_(i,j)^(s,d) - \sum_{(i,j)\el E} x_(j,i)^(s,d) = 
\begin{cases}
    1 &\text{if i=s} \\
    -1 &\text{if i=d} &\forall s,d,i \el V \\
    0 &\text{otherwise}
\end{cases}
\end{align} 
\end{document}

I am almost sure the sub- and superscripts will not look nice this way. But the problem is also: I get an error saying "undefined control sequence \end{align}" In order to be able to use the align-environment I should only need to import amsmath, is that correct?

Answer 1

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{gather*}
\sum\limits_{(i,j)\in E}x_{i,j}^{s,d}-\sum\limits_{(j,i)\in E}x_{j,i}^{s,d} = 
    \begin{cases} 
        1 & \mbox{if } i = s \\
        -1 & \mbox{if } i = d \\
        0 & \text{otherwise} 
    \end{cases} 
    \forall s,d,i \in V \\
\sum\limits_{(s,d)}x_{i,j}^{s,d}t^{s,d}\leq \sum\limits_{n\in [1\ldots N]}K_{n}l^{n}_{i,j} 
\qquad 
\forall (i,j)\in E 
\end{gather*} 
\end{document}

I would like to add to the previous answer that instead of align, using gather* environment will allow you to center both equations and to print them without any number marks.

Answer 2

perhaps something like

\documentclass{article}
\usepackage{amsmath}
\begin{document}
% no alignment
\begin{gather}
\sum_{(i,j)
% \in not \el
\in
 E}
% {} not () 
x_{i,j}^{s,d} - \sum_{(i,j)\in E}
% {} not ()
x_{j,i}^{s,d} = 
\begin{cases}
%  math around equation
    1 &\text{if $i=s$}\\
% \in not \el
    -1 &\text{if $i=d \quad\forall s,d,i \in V$}\\
    0 &\text{otherwise}
\end{cases}\\
\sum_{\ldots} \leq \sum_{\ldots}
\end{gather} 
\end{document}

The second inequality just a sketch of course.

Answer 3

I propose this variant layout, with mathtools (needless to load amsmath):

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{alignat*}{2}
\smashoperator[r]{\sum_{(i,j)\in E}}x_{i,j}^{s,d}-\smashoperator{\sum_{(j,i)\in E}}x_{j,i}^{s,d} & =
    \begin{cases}
        \phantom{-}1 & \mbox{if } i = s \\
        -1 & \mbox{if } i = d \\
        \phantom{-}0 & \text{otherwise}
    \end{cases}
     &\qquad & \forall s,d,i \in V \\[1.5ex]
\sum_{(s,d)}x_{i,j}^{s,d}t^{s,d} & \leq \smashoperator{\sum_{n\in [1\ldots N]}}K_{n}l^{n}_{i,j}
 & &
\forall (i,j)\in E
\end{alignat*}

\end{document}