Hiding the dividing line in a fraction [duplicate]

Question

I would like to be able to hide the dividing line in the fraction that arises from applying a command such as \dfrac.

In particular, what I would like to do is take an expression such as the following:

\dfrac{\partial^2 f}{\partial x^2} + \dfrac{\partial^2 f}{\partial y^2}

and create a duplicate of it which highlights only the fs. I can use \phantom to hide other symbols that occur in the partial derivatives, but I don't know how to use it to hide the fraction bar.

Could anybody suggest a way forward?

Answer 1

You could employ array environments, or you could define a macro called, say, \nolinefrac that employs the low-level \genfrac macro that's provided by the amsmath package.

\documentclass{article}
\usepackage{amsmath} % for \dfrac, \genfrac, and \text macros
\newcommand{\nolinefrac}[2]{\genfrac{}{}{0pt}{}{#1}{#2}}

\begin{document}

\[
\frac{\partial^2 f}{\partial x^2} 
+ 
\frac{\partial^2 f}{\partial y^2}
\quad\text{vs.}\quad
\begin{array}{@{}c@{}}
   \partial^2 f \\[0.5ex] \partial x^2
\end{array}
+
\begin{array}{@{}c@{}}
   \partial^2 f \\[0.5ex] \partial y^2
\end{array}
\quad\text{vs.}\quad
\nolinefrac{\partial^2 f}{\partial x^2} 
+ 
\nolinefrac{\partial^2 f}{\partial y^2}
\]

\end{document}