How to write this expression?

Question

How can I write the vertical line in this expression? It is as tall as the before relation.

Answer 1

You can do

\documentclass{article}
\begin{document}
\begin{equation}
    4\pi\epsilon^{2}\frac{d\tilde{G}}{dR}\biggr|_{R=\epsilon}.
\end{equation}
\end{document}

Some more details on the use of \bigl ... can be found in this post.

Answer 2

You can use the esdiff package, for a simpler typing of derivatives (partial or not) of any order in Leibniz notation. For derivatives evaluated at some point, you have a \diff*command with 3 arguments, the function, the variable and the evaluation point, but it uses a pair of parentheses. If you want a single vertical line, you have to do it by hand. Here is an example of both:

\documentclass{article}
\usepackage{mathtools}
\usepackage{esdiff} 

\begin{document}

\[ 4\pi\varepsilon ^2\diff*{\widetilde{G}}{R}{R = \varepsilon } \]%

\[ 4\pi\varepsilon ^2\diff{\widetilde{G}}{R}\biggm\vert_{R = \varepsilon } \]%

\end{document} 

Answer 3

If you prefer to have the same image you inserted with the same symbols, you can also use my code (with slanted operator "d" or vertical mode for "d")

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}        
\usepackage{times}
\usepackage{newtxtext,newtxmath}
\usepackage{fancybox}
\usepackage{xcolor}
\begin{document}
\[\colorbox{gray!20}{$4\pi\varepsilon^2\dfrac{d\tilde{G}}{dR}\biggr|_{R=\varepsilon}.$}\]

\[\colorbox{gray!20}{$4\pi\varepsilon^2\dfrac{\mathrm{d}\tilde{G}}{\mathrm{d}R}\biggr|_{R=\varepsilon}.$}\]

\end{document}