[Tex/LaTex] Inexplicable equation numbering in Beamer

beamerequations

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?

Best Answer

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}
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