Problem with using pause and onslide in one frame

Question

I'm trying to use \pause in align mode but it's not working. I ended up using onslide which it works fine. However, when I use both of them in one frame, some of onslide work while the other are not. This is a minimal example to regenerate the problem.

\documentclass[t]{beamer}

\usepackage{amsmath}

\begin{document}

\begin{frame}
AAAAA CCCCCC \\
\pause 
AAAAA CCCCCC
\begin{align*}
    \onslide<1->{a &= b \\}
    \onslide<2->{b &= c \\}
    \onslide<3>{ a &= c}
\end{align*}
\end{frame}

\end{document}

As you can see, the first two lines show up together. While the third line appears in the next slide. When I comment out \pause, the problem goes away. How to overcome this issue?

Answer 1

The \pause command does exactly what it says: stops the frame where it is. As such, if you then specify slide 1 later on in the frame, that material will appear with slide 2, as you've observed. I strongly advise not mixing \pause with any more complex overlay control: \pause is simple but limited, other overlay specs are more complicated but as a result more powerful. Sam Carter's answer shows that if you want the align to appear after the text, it's easy with a mix of approaches. If you want the two to appear at the same time, I'd go with something like

\documentclass[t]{beamer}

\usepackage{amsmath}

\begin{document}

\begin{frame}
\onslide<+->{AAAAA CCCCCC} \\
\onslide<+->{AAAAA CCCCCC}
\begin{align*}
    \onslide<.(-1)->{a &= b \\}
    \onslide<.->{b &= c \\}
    \onslide<+->{ a &= c}
\end{align*}
\end{frame}

\end{document}

(See Relative overlay specification in beamer? for more on relative overlay specifications.)

Answer 2

The "problem" is, that the first pause already creates an overlay, so you have to start counting at 2 in the align and not at 1. But to make this more flexible, simply don't use hard coded numbers but relative overlays.

\documentclass[t]{beamer}

\usepackage{amsmath}

\begin{document}

\begin{frame}
AAAAA CCCCCC \\
\pause 
AAAAA CCCCCC
\begin{align*}
    \onslide<+->{a &= b \\}
    \onslide<+->{b &= c \\}
    \onslide<+>{ a &= c}
\end{align*}
\end{frame}

\end{document}