In math mode, how do I make the fraction bar "invisible"?

Question

I am trying to typeset a "fraction" without the fraction bar. I have tried to do this using the genfrac, but as in the example below, the numerator and denominator in genfrac are farther apart than in frac. How can I correct this?

I'm trying to create a table in which one of the entries ("cells") contains two entries -- one on top of the other -- as in row 5 in the MWE below. In other words, I want to make the fraction bar "invisible" in the MWE table below.

\documentclass[oneside,11pt]{book}
\usepackage[semibold,tt=false]{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}
\usepackage[
  expansion = false ,
  tracking = smallcaps ,
  letterspace = 40 ,
  final
]{microtype}
\usepackage{booktabs}
\usepackage[font={sf,small},labelsep=quad,labelfont=sc]{caption}
\usepackage[subrefformat=parens]{subcaption}
\begin{document}
$\frac{1}{2};\genfrac{}{}{0pt}{}{1}{2}$
\begin{table}[!h]
  \begin{tabular}{lll}
    Row 1 & 1.23 & Some text.\
    Row 2 & 7.89 & Additional text.\
    Row 3 & 4.56 & More text.\
    \addlinespace
    Row 4 & 1.23 & Some text.\
    Row 5 & $\frac{\textit{A}:;3.14}{\textit{B}:;6.28}$ & Additional text.\
    Row 6 & 4.56 & More text.\
  \end{tabular}
\end{table}
\end{document}

Answer 1

The \frac macro is defined as \begingroup #1\endgroup \over #2 (for the definition enter texdef -t latex \frac in a terminal). You can redefine this to print the \over line in white:

\newcommand{\fracnoline}[2]{{\begingroup #1\endgroup \color{white}\over\color{black} #2}}

As Mico noted in comments, to generalize the macro in case the original color is not black you can use the following definition:

\newcommand{\fracnoline}[2]{\colorlet{foo}{.}{\begingroup #1\endgroup \color{white}\over\color{foo} #2}}

Full code:

\documentclass[oneside,11pt]{book}
\usepackage[semibold,tt=false]{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}
\usepackage[
  expansion = false ,
  tracking = smallcaps ,
  letterspace = 40 ,
  final
]{microtype}
\usepackage{booktabs}
\usepackage[font={sf,small},labelsep=quad,labelfont=sc]{caption}
\usepackage[subrefformat=parens]{subcaption}
\usepackage{xcolor}
\begin{document}
\newcommand{\fracnoline}[2]{\colorlet{foo}{.}{\begingroup #1\endgroup \color{white}\over\color{foo} #2}}
$\frac{1}{2};\genfrac{}{}{0pt}{}{1}{2}$
\begin{table}[!h]
  \begin{tabular}{lll}
    Row 1 & 1.23 & Some text.\
    Row 2 & 7.89 & Additional text.\
    Row 3 & 4.56 & More text.\
    \addlinespace
    Row 4 & 1.23 & Some text.\
    Row 5 & $\frac{\textit{A}:;3.14}{\textit{B}:;6.28} \fracnoline{\textit{A}:;3.14}{\textit{B}:;6.28}$ & Additional text.\
    Row 6 & 4.56 & More text.\
  \end{tabular}
\end{table}
\end{document}

Result:

Answer 2

That's a bit unexpected, I'll have to read appendix G again... but \genfrac is just exposing the underlying primitive and \atop and \above0pt both omit the rule but increase the height of the numerator. You can specify the negative width of the default rule (-0.4pt) to get the same height as \frac (which uses the \over primitive).

The first four forms here use the same height as\frac, the last three are higher.

Note this only applies to textstyle not displaystyle.

\documentclass[oneside,10pt]{book}
\usepackage{amsmath}
\usepackage[T1]{fontenc}
\begin{document}
\hrule
$
\frac{1}{2};
\genfrac{}{}{-.4pt}{}{1}{2};
{1 \above -.4pt 2};
{1\over 2};
{1 \above 0pt 2};
\genfrac{}{}{0pt}{}{1}{2};
{1\atop 2}
$
\hrule
\smallskip
\hrule
$\displaystyle
\frac{1}{2};
\genfrac{}{}{-.4pt}{}{1}{2};
{1 \above -.4pt 2};
{1\over 2};
{1 \above 0pt 2};
\genfrac{}{}{0pt}{}{1}{2};
{1\atop 2}
$
\hrule
\end{document}

Answer 3

With amsmath you can use the \substack command:

\documentclass[oneside,11pt]{book}
\usepackage{amsmath}
\usepackage{booktabs}
\begin{document}
\begin{table}[!h]
   \begin{tabular}{lll}
    Row 1 & 1.23 & Some text.\
    Row 2 & 7.89 & Additional text.\
    Row 3 & 4.56 & More text.\
    \addlinespace
    Row 4 & 1.23 & Some text.\
    Row 5 & $ \substack{\textit{A}:;3.14\[1pt]\textit{B}:;6.28}$ & Additional text.\
    Row 6 & 4.56 & More text.\
  \end{tabular}
 \end{table}
\end{document}

Answer 4

Starting from David's answer, a fixed version that works in display, text and script style (not at scriptscript style, where other optimizations are used).

\documentclass[oneside,10pt]{book}
\usepackage{amsmath}
\usepackage[semibold,tt=false]{libertine}
\usepackage{libertinust1math}
\usepackage[T1]{fontenc}
\makeatletter
\DeclareRobustCommand{\xabove}[2]{{%
  \mathpalette\fix@genfrac{{#1}{#2}}%
}}
\newcommand{\fix@genfrac}[2]{\fix@@genfrac#1#2}
\newcommand{\fix@@genfrac}[3]{%
  \genfrac{}{}{\fix@@@genfrac{#1}}{}{#2}{#3}%
}
\newcommand{\fix@@@genfrac}[1]{%
  \ifx#1\displaystyle 0pt\else
  \ifx#1\textstyle -\fontdimen8\textfont2 \else
  \ifx#1\scriptstyle -\fontdimen8\scriptfont2 \else
  0pt % but doesn't really work :-(
  \fi\fi\fi
}
\makeatother
\begin{document}
$\displaystyle\frac{1}{2}\xabove{1}{2}$
$\textstyle\frac{1}{2}\xabove{1}{2}$
$\scriptstyle\frac{1}{2}\xabove{1}{2}$
%$\scriptscriptstyle\frac{1}{2}\xabove{1}{2}$
\end{document}