eqnarray with subnumber

Question

When I use eqnarray

\begin{eqnarray}

eq\\
eq\\
eq\\

\end{eqnarray}

this show me

eq (1)
eq (2)
eq (3)

In some papers I see this output

eq (1a)
eq (1b) 
eq (1c) 

how I can make this way ?

Answer 1

Avoid eqnarray! Use the align environment inside the subequations environment from the amsmath package.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}   % loads »amsmath«

\begin{document}
  \begin{subequations}
    \begin{align}
      (a+b)^2 &= a^2+2ab+b^2 \\
      (a-b)^2 &= a^2-2ab+b^2  \\
      (a+b)(a-b) &= a^2-b^2 
    \end{align}
  \end{subequations}
\end{document}

Answer 2

amsmath has an environment subequations that will do what you want:

\begin{subequations}
\begin{eqnarray}
 eq \\
 eq \\
 eq
\end{eqnarray}
\end{subequations}

some observations:

• you don't want blank lines withn the scope of display math; they will result in error messages.
• you don't want \\ at the end of the last line, or you'll end up with too much space below the display.
• it would really be better to use one of the multi-line display structures provided by amsmath rather than eqnarray (see this article for the reasons why).

to find out what structures are provided by amsmath, if you have a tex live installation, type texdoc amsmath at a command line prompt.

Answer 3

The second look (with a, b, c ... appended to the "main" equation number) can be achieved with the subequations environment of the amsmath package. The following MWE (minimum working example) demonstrates the basic usage of this package:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{subequations}
\begin{align}
a &= b\\
c &= d\\
e &= f
\end{align}
\end{subequations}
\end{document}

Finally: Don't use the eqnarray environment -- use the align environment instead. The eqnarray environment has several severe shortcomings; for a justification of this assertion see, for instance, "\eqnarray vs \align".