Fraction with doubled line

Question

Is there a way to make the fraction line look like the equality (=) sign, i.e., to have two parallel horizontal lines with a little gap in between? Many thanks!

Answer 1

Use either \Tfrac or \Dfrac (short for Text and Display style "equal fraction") in the following:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\newcommand{\Tfrac}[2]{%
  \ooalign{%
    $\genfrac{}{}{1.2pt}1{#1}{#2}$\cr%
    $\color{white}\genfrac{}{}{.4pt}1{\phantom{#1}}{\phantom{#2}}$}%
}
\newcommand{\Dfrac}[2]{%
  \ooalign{%
    $\genfrac{}{}{1.2pt}0{#1}{#2}$\cr%
    $\color{white}\genfrac{}{}{.4pt}0{\phantom{#1}}{\phantom{#2}}$}%
}
\begin{document}
$f(x)=\Tfrac{1}{2}+\frac{1}{2}$
\end{document}

For a short course on \ooalign, see \subseteq + \circ as a single symbol (“open subset”).

---

Here is a slightly modified set of commands: \Efrac and \efrac. While both adjust their fraction size based on the math style automatically (using \mathchoice), the former is vertically higher (set based on the height of the two outer black fraction lines) than the latter (set based on the height of a regular fraction).

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\newcommand{\Efrac}[2]{%
  \mathchoice
    {\ooalign{%
      $\genfrac{}{}{1.2pt}0{#1}{#2}$\cr%
      $\color{white}\genfrac{}{}{.4pt}0{\phantom{#1}}{\phantom{#2}}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}1{#1}{#2}$\cr%
      $\color{white}\genfrac{}{}{.4pt}1{\phantom{#1}}{\phantom{#2}}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}2{#1}{#2}$\cr%
      $\color{white}\genfrac{}{}{.4pt}2{\phantom{#1}}{\phantom{#2}}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}3{#1}{#2}$\cr%
      $\color{white}\genfrac{}{}{.4pt}3{\phantom{#1}}{\phantom{#2}}$}}%
}
\newcommand{\efrac}[2]{%
  \mathchoice
    {\ooalign{%
      $\genfrac{}{}{1.2pt}0{\hphantom{#1}}{\hphantom{#2}}$\cr%
      $\color{white}\genfrac{}{}{.4pt}0{\color{black}#1}{\color{black}#2}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}1{\hphantom{#1}}{\hphantom{#2}}$\cr%
      $\color{white}\genfrac{}{}{.4pt}1{\color{black}#1}{\color{black}#2}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}2{\hphantom{#1}}{\hphantom{#2}}$\cr%
      $\color{white}\genfrac{}{}{.4pt}2{\color{black}#1}{\color{black}#2}$}}%
    {\ooalign{%
      $\genfrac{}{}{1.2pt}3{\hphantom{#1}}{\hphantom{#2}}$\cr%
      $\color{white}\genfrac{}{}{.4pt}3{\color{black}#1}{\color{black}#2}$}}%
}
\begin{document}
\[ f(x)={\textstyle\Efrac{1}{2}+\frac{1}{2}} \sim \Efrac{3}{4}+\frac{3}{4} \]
\[ f(x)={\textstyle\efrac{1}{2}+\frac{1}{2}} \sim \efrac{3}{4}+\frac{3}{4} \]
\end{document}

Answer 2

A solution without colors, which could break in case of colored background.

The fraction is typeset twice: the first time it is raised and the denominator is a phantom; the second time it is lowered and the numerator is a phantom. The two are superimposed with the help of \ooalign.

The gap is customizable with the parameter \doublefracgap.

\documentclass{article}
\usepackage[fleqn]{amsmath}

\makeatletter
\newlength{\doublefracgap}
\setlength{\doublefracgap}{0.75pt}
\DeclareRobustCommand{\doublefrac}[2]{%
  \mathinner{\mathpalette\doublefrac@{{#1}{#2}}}%
}
\newcommand{\doublefrac@}[2]{\doublefrac@@#1#2}
\newcommand{\doublefrac@@}[3]{%
  \ooalign{%
    \raisebox{\doublefracgap}{$\m@th#1\frac{#2}{\phantom{#3}}$}\cr
    \raisebox{-\doublefracgap}{$\m@th#1\frac{\phantom{#2}}{#3}$}\cr
  }%
}
\newcommand{\ddoublefrac}[2]{{\displaystyle\doublefrac{#1}{#2}}}
\newcommand{\tdoublefrac}[2]{{\textstyle\doublefrac{#1}{#2}}}
\makeatother

\setlength{\mathindent}{0pt} % just for the example
\setlength{\parindent}{0pt} % just for the example

\begin{document}

\[
\doublefrac{1}{2}\ne\frac{1}{2} \qquad \sqrt{\doublefrac{x^2+2}{x^2-1}}
\]
\[
\frac{1}{2}\ne\doublefrac{1}{2}
\]
$\doublefrac{1}{2}\ne\frac{1}{2}$

$\frac{1}{2}\ne\doublefrac{1}{2}$

$\ddoublefrac{1}{2}\ne\dfrac{1}{2}$

$\dfrac{1}{2}\ne\ddoublefrac{1}{2}$

\end{document}