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 ?
[Tex/LaTex] eqnarray with subnumber
amsmathequationsnumbering
Related Solutions
The »mathtools« package provides the macros \newtagform and \usetagform which allow to do this quite comfortable.
\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools} % loads »amsmath«
\newtagform{dots}{\ldots(}{)}
\begin{document}
\begin{equation}
(a+b)^2 = a^2+2ab+b^2 \label{eqn:binomi-one}
\end{equation}
\begin{equation}
(a-b)^2 = a^2-2ab+b^2 \label{eqn:binomi-two}
\end{equation}
\begin{equation}
\usetagform{dots}
(a+b)(a-b) = a^2-b^2 \label{eqn:binomi-three}
\end{equation}
%
The Binomi equations~\eqref{eqn:binomi-one},~\eqref{eqn:binomi-two} and~\eqref{eqn:binomi-three}.
\end{document}
Works of course also with the numbering scheme for equations in report and book.
The output of eqnarray is wrong anyway and there's little to do about it. Consider also that it doesn't work with cross-references if hyperref or cleveref are involved.
No, there's no reason for using it.
The first example can be dealt with using array:
\documentclass{article}
\usepackage{amsmath,array}
\begin{document}
\begin{equation*}
\renewcommand{\arraystretch}{1.5}
\begin{array}{@{} r @{} >{{}} c <{{}} @{} l @{} }
2^n
& = & \underbrace{2 \cdot 2 \cdot \ldots \cdot 2}_{n \text{ times}}\\
& \overset{n>3}{=} & 2 \cdot 2 \cdot 2 \cdot 2 \cdot \underbrace{2 \cdot 2 \cdot \ldots \cdot 2}_{n - 4 \text{ times}}\\
& = & 16 \cdot \underbrace{2 \cdot 2 \cdot \ldots \cdot 2}_{n - 4 \text{ times}}\\
& < & 24 \cdot \underbrace{2 \cdot 2 \cdot \ldots \cdot 2}_{n - 4 \text{ times}}\\
& = & 1 \cdot 2 \cdot 3 \cdot 4 \cdot \underbrace{2 \cdot 2 \cdot \ldots \cdot 2}_{n - 4 \text{ times}}\\
& \overset{n>3}{\leq} & 1 \cdot 2 \cdot 3 \cdot 4 \cdot 5 \cdot \ldots \cdot n\\
& = & n!
\end{array}
\end{equation*}
A better proof, by induction, is
\begin{alignat*}{2}
2^4 &= 16 < 4! = 24 \\
2^{n+1} &= 2\cdot 2^n &\quad&\text{for $n>3$} \\
&< 2\cdot n! &\quad&\text{induction hypothesis}\\
&< (n+1)\cdot n! \\
&= (n+1)!
\end{alignat*}
\end{document}
The second example is even easier: use a good alignment point:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\sigma =[ &x_1/t_1,x_2/t_2,\\
& x_3/t_3,x_4/t_4]
\end{align*}
\end{document}
More lines can be accommodated:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\sigma = [ &x_1/t_1,x_2/t_2, \\
& x_3/t_3,x_4/t_4] \\
\tau = [ &y_1/t_1] \\
\psi = [ &x_1/t_1,x_2/t_2,\\
& x_3/t_3,x_4/t_4]
\end{align*}
\end{document}
Best Answer
amsmathhas an environmentsubequationsthat will do what you want:some observations:
\\at the end of the last line, or you'll end up with too much space below the display.amsmathrather thaneqnarray(see this article for the reasons why).to find out what structures are provided by
amsmath, if you have a tex live installation, typetexdoc amsmathat a command line prompt.