CODEWITHSUNDEEP
latexequationalignment
Tim
Tim

Reputation: 99526

number several equations with only one number

How can I number several equations in a align environment using only one number?

For example

\begin{align}
w^T x_i + b \geqslant 1-\xi_i \text{ if } y_i=1, \nonumber \\
w^T x_i + b \leqslant -1+\xi_i \text{ if } y_i=-1,
\end{align}

The numbering will appear next to the second equation. But it would be better if it appears between the lines of the two equations.

In this case how to label this group of equations for later referring to?

Thanks and regards!

Upvotes: 57

Views: 140074

Answers (3)

Ammar N. Abbas
Ammar N. Abbas

Reputation: 63

Gathered environment can be used as well to have everything centered.

\begin{equation}
  \begin{gathered}
  w^T x_i + b &\geq 1-\xi_i &\text{ if }& y_i=1,  \\
  w^T x_i + b &\leq -1+\xi_i & \text{ if } &y_i=-1,
  \end{gathered}
\end{equation}

Upvotes: 1

Bart Kiers
Bart Kiers

Reputation: 170268

How about something like:

\documentclass{article}

\usepackage{amssymb,amsmath}

\begin{document}

\begin{equation}\label{A_Label}
  \begin{split}
    w^T x_i + b \geqslant 1-\xi_i \text{ if } y_i &= 1, \\
    w^T x_i + b \leqslant -1+\xi_i \text{ if } y_i &= -1
  \end{split}
\end{equation}

\end{document}

which produces:

enter image description here

Upvotes: 16

Aniko
Aniko

Reputation: 18874

First of all, you probably don't want the align environment if you have only one column of equations. In fact, your example is probably best with the cases environment. But to answer your question directly, used the aligned environment within equation - this way the outside environment gives the number:

\begin{equation}
  \begin{aligned}
  w^T x_i + b &\geq 1-\xi_i &\text{ if }& y_i=1,  \\
  w^T x_i + b &\leq -1+\xi_i & \text{ if } &y_i=-1,
  \end{aligned}
\end{equation}

The documentation of the amsmath package explains this and more.

Upvotes: 83

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