# “Parallel reduction arrow”: -//->

What's the best way to get a "parallel reduction arrow" (as in the one used to show confluence of the $\lambda$-calculus, if that means anything)

Concretely I want an arrow with a // crossing the middle of it. Using || could also be acceptable.

Right now I have:

\newcommand{\rpbeta}{\mkern-7mu\centernot{\mkern7mu\centernot\rbeta}}


where \rbeta is $\longrightarrow_\beta$.

Giving something kind of OK, but spacing seems to vary each time I use it.

• What is \rbeta, please? – yo' Feb 5 '17 at 19:45
• Ah, sorry, $\longrightarrow_\beta$. – Guido Feb 5 '17 at 19:56

I would use my favourite \mathcenterto{#1}{#2} construct which allows you to center #1 over #2. One additional \mathclap makes the right-side slash centered and the left-side one adjacent to it.

\documentclass{article}

\pagestyle{empty}

\usepackage{mathtools}

\newcommand*\mathcenterto[2]{\mathclap{\phantom{#2}\mathclap{#1}}{#2}}
\newcommand*\rpbeta{\mathrel{\mathcenterto{\mathclap{/}/}{\longrightarrow}}_\beta}

\begin{document}

$A \rpbeta B _{A \rpbeta B _{A \rpbeta B _{A \rpbeta B }}}$

\end{document}


You can emulate the working of \centernot:

\documentclass{article}

\makeatletter
\newcommand{\rpgeneric}[1]{\mathpalette\rp@generic{#1}}
\newcommand{\rp@generic}[2]{%
\mathrel{%
\rlap{%
\settowidth\dimen@{$\m@th#1{\longrightarrow}$}%
\kern.55\dimen@
\settowidth\dimen@{$\m@th#1=$}%
\kern-.53\dimen@
$\m@th#1\not$%
\kern-.3\dimen@
$\m@th#1\not$%
}%
{\longrightarrow_{#2}}%
}%
}
\makeatother
\newcommand{\rpbeta}{\rpgeneric{\beta}}

\begin{document}

$\rpbeta_{\rpbeta}$

\end{document}


In addition to the mathtools package, I would use amsmath.txfonts with the following code (however the fonts could create some other problems elesewhere in the text...)

\documentclass{article}
\usepackage{amsmath,txfonts}
\usepackage{mathtools}
\begin{document}
$\mathrlap{\longrightarrow}\varparallel$
\end{document}


• Have you tried to actually use it? Like in $A\mathrlap{\longrightarrow}\varparallel B$? – yo' Feb 5 '17 at 23:09
• Yeah, spacing looks a bit bad on this one. – Guido Feb 5 '17 at 23:28
• I really don't understand how can someone upvote this answer. It's plain wrong as the spacing is completely off. This is unusable. – yo' Feb 6 '17 at 11:53
• Note also that mathtools loads amsmath, so in your case having amsmath is redundant. – Torbjørn T. Feb 6 '17 at 12:05
• Besides breaking the spacing, loading txfonts will change the appearance of the whole document. You wouldn't rebuild a house just for getting a new window, would you? – egreg Feb 6 '17 at 13:00