How can I number a few equations together

Question

i would like to write few equations, but all the equations will get only once numbered and in the middle (right side) of the equations. e.q. if i write 3 equations, the number will be to the right of the second equation line and if i have 2 equations, then the number will between the two equations and at the right side.

in addition i would like to have something like "&" (align) that i could align my equations at certain points.

Example:

a   = x_{ij}
                  (1)
b_j = y_j      

where of course the number of equation doesn't take a all line space.

Answer 1

The amsmath package provides the split environment. You can use a split environment inside an align environment or inside an equation environment; a little example:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
\begin{split}
  a   &= x_{ij} \\
  b_j &= y_j 
\end{split}
\end{align}

\end{document}

Answer 2

The amsmath package has many facilities, among which the aligned environment that does similarly to align, but produces a block usable in a bigger formula:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{aligned}
  a   &= x_{ij} \\
  b_j &= y_j 
\end{aligned}
\end{equation}

\end{document}

What's the difference with split? That aligned, like align, allows many alignment points.

Answer 3

Although amsmath functionality is preferred, there are a multitude of ways this can be achieved. Here's one using only an array:

\documentclass{article}
\begin{document}
\begin{equation}
  \renewcommand{\arraystretch}{1.2}% To spread out the equations
  \begin{array}{r@{\;}l}
    a =& x_{ij} \\
    b_j =& y_j
  \end{array} \label{eq}
\end{equation}
\end{document}

It has the same layout - in terms of input - to the align environment. Moreover, it now also straight-forward to combine the equations using a brace (say) - achieved using a \left. and \right\} pair:

\documentclass{article}
\begin{document}
\begin{equation}
  \renewcommand{\arraystretch}{1.2}% To spread out the equations
  \left.\begin{array}{r@{\;}l}
    a =& x_{ij} \\
    b_j =& y_j
  \end{array}\right\} \label{eq}
\end{equation}
\end{document}

rcases (from mathtools) also provides the above functionality, but is usually intended for a different purpose. As such, the spacing/alignment is not as expected. One can use the undocumented \newcases to define a comparable alternative. Below I've adapted rcases to have a right-aligned first column, and introduce a \thickmuskip between the two "columns", similar to what one would expect around ordinary and relation atoms:

\documentclass{article}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools
\makeatletter
\newcases{Rcases}{$\mskip\thickmuskip$}{%
  \hfil$\m@th{##}$}{$\m@th{##}$\hfil}{.}{\rbrace}
\makeatother
\begin{document}
\begin{equation}
  \begin{Rcases}
    a   &= x_{ij} \\
    b_j &= y_j
  \end{Rcases} \label{eq}
\end{equation}
\end{document}