Inexplicable equation numbering in Beamer

Question

Here is my code:

\documentclass{beamer}
\usepackage{amsfonts}
\usepackage{amsmath}

\begin{document}
\frame{\titlepage}

\begin{frame}
    \begin{gather}\label{equation1}
    a^2 + b^2 = c^2
    \end{gather}
    Consider \eqref{equation1}. This is fine.
\end{frame}

\begin{frame}
    But then:
    \begin{gather}\label{equation2}
    x^2 + y^2 = z^2\nonumber\\
    r + s + t
    \end{gather}

    \begin{equation}
    x = 2
    \end{equation}

    Equation (2) is referenced as \eqref{equation2}. Why???
\end{frame}

\end{document}

I do not understand why in the last line, \eqref(equation2) shows up as (3) despite clearly being designated as (2). How do I work around this in an efficient manner, particularly since I am going to have to code multiple slides with this sort of thing?

You explicately said that the second equation will not get a number. This would confuse me as well. Place the label at the end of the gather block, not at the beginning. — Johannes_B, Feb 12 '17 at 08:21
Aside: Using gather (and align) environments for single-line displayed equations constitutes a form of typographic abuse. Use equation environments instead. See the posting What are the differences between $$, [, align, equation and displaymath for more information on this subject. — Mico, Feb 12 '17 at 08:28
Add your \label after the second equation of your gather environment. — Paul Gaborit, Feb 12 '17 at 08:30
I put the label just inside the \end{gather} tag. It does what I want now. But why was it messing up to begin with? Again, this was not a problem in regular documents. — Forklift17, Feb 12 '17 at 08:30

score 6 · Answer 1 · answered Feb 12 '17 at 09:07

You have the \label in the wrong place: they need to come after the numbered item:

\documentclass{beamer}
\usepackage{amsfonts}
\usepackage{amsmath}

\begin{document}
\frame{\titlepage}

\begin{frame}
    \begin{gather}
    a^2 + b^2 = c^2\label{equation1}
    \end{gather}
    Consider \eqref{equation1}. This is fine.
\end{frame}

\begin{frame}
    But then:
    \begin{gather}
    x^2 + y^2 = z^2\nonumber\\
    r + s + t\label{equation2}
    \end{gather}

    \begin{equation}
    x = 2
    \end{equation}

    Equation~(2) is referenced as \eqref{equation2}. Why???
\end{frame}

\end{document}

score 5 · Accepted Answer · answered Feb 12 '17 at 09:27

Amsmath environments align and gather are intended to typeset multiple equations, so they require putting one \label{whatever} per line you want to refer to.

What happens if you put \label{equation2} at the line which is marked by \nonumber (so, you don't want to number this equation) is that the label will pick up the previously stepped counter. In this case it's a frame number counter, so your (3) meant the frame number and not the equation number.

Moving he label \label{equation2} definition to the second equation in the gather environment helps.

\documentclass{beamer}
\usepackage{amsfonts}
\usepackage{amsmath}

\begin{document}
\frame{\titlepage}

\begin{frame}
    \begin{gather}\label{equation1}
    a^2 + b^2 = c^2
    \end{gather}
    Consider \eqref{equation1}. This is fine.
\end{frame}

\begin{frame}
    But then:
    \begin{gather}
    x^2 + y^2 = z^2\nonumber\\
    r + s + t\label{equation2}
    \end{gather}

    \begin{equation}
    x = 2
    \end{equation}

    Equation (2) is referenced as \eqref{equation2}. Why???
\end{frame}

\end{document}

Good catch, identifying the fact that the unexpected number "3" refers to the frame number and not to the (later) equation number. — Mico, Feb 12 '17 at 10:38

Inexplicable equation numbering in Beamer

2 Answers2