[Tex/LaTex] Equivalent symbol not working and subscript on vertical line

beamerdelimitersmath-modesubscripts

I have couple of issues here.

I am making a presentation on beamer and I am using the following code:

\begin{align*}
    & F_{in} = \int \frac{\partial Q}{\partial s}|_{in} ds, 
              F_{out} = \int \frac{\partial Q}{\partial s}|_{out} ds \\
    & S_{in} \equiv \frac{F_{in}}{Q_{in}}, 
              S_{out} \equiv \frac{F_{out}}{Q_{out}}
\end{align*}

However, the result of \equiv I am getting is := and I would also appreciate if someone points how to make something like this in LaTeX |_{out} to fit with the equation height.

Best Answer

This might be what you're after:

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\documentclass{beamer}% http://ctan.org/pkg/beamer
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\begin{align*}
  F_\text{in} &= \int \left.\kern-\nulldelimiterspace\frac{\partial Q}{\partial s}\right|_\text{in} ds & 
    F_\text{out} &= \int \left.\kern-\nulldelimiterspace\frac{\partial Q}{\partial s}\right|_\text{out} ds \\
  S_\text{in} &\equiv \frac{F_\text{in}}{Q_\text{in}} & 
    S_\text{out} &\equiv \frac{F_\text{out}}{Q_\text{out}}
\end{align*}
\end{document}

The extensible \right| is used, together with a (blank) \left. pair. However, to avoid unnecessary spacing of the empty \left delimiter, a negative \kern is used.

Since in and out (most likely) are text components in the set of equations, use amsmath's \text.

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