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I currently use two relation symbols as shown below. However, as I need to put in quite a bit of information above and, in the first case, also below the arrow, I'd like to have longer arrows. In my setting the example below already shows the longest possible items I need to put above/below.

It would be nice to have \tau\tau' on the line of the arrow, with the arrow head strictly to the right. A bit more line extending to the left would also be nice.

One idea is to horizontally stretch the arrow symbol. However, I guess this would also stretch the arrow head. Another idea might be to combine several symbols - as creating a long line should be doable.

However, I am not experienced with this and would like to read your input on this.

arrows

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How do you get the rotated pitchfork in the second row? Do you really need the triangular head for the arrow? –  egreg May 3 at 10:29
1  
Using a minus sign followed by a negative kern, -\kern-3pt\rightarrow for example, is a standard approach. The amount of the kern might need adjustment, and perhaps \mkern with units of "mu" would be preferable. –  Steven B. Segletes May 3 at 10:30
    
egreg, this is \leftpitchfork from MnSymbols. Semantically I use it as a \to b or a \in b, so replacing the pitchfork by a proper \in might be nice, too. I do not really care about the looks of the first arrow, but I already use several other arrows. Thus, to make this relation unique, I need a new symbol. Having another, non-standard, alternative might also work out just fine. –  C-Otto May 3 at 10:33
    
Steven, with -\kern-1.1ex\leftpitchfork this looks about right. However, in bold math mode the - line looks bigger than the line of \leftpitchfork. I might live with that, though. –  C-Otto May 3 at 10:37
1  
If you want me to be notified of your return comment, you need to put @Steven B. Segletes in your comment. As to the bold math issue, you could vertically stack two minuses nearly on top of each other to provide additional width, or else use a \rule of the appropriate thickness. Or even a \roundrule! (see tex.stackexchange.com/questions/161297/…) –  Steven B. Segletes May 3 at 12:55

2 Answers 2

up vote 6 down vote accepted

I'm not a fan of MnSymbol. Here's a set of macros without it; the pitchfork is obtained by rotating the symbol in amssymb:

\documentclass{article}
\usepackage{amsmath,amssymb,graphicx,stmaryrd}

\newcommand{\rightpitch}{%
  \mathrel{% it's a relation
    \text{% scaled according to math style
      \smash{\raisebox{-.225ex}{% lowered a bit
        \rotatebox[origin=c]{90}{$\pitchfork$}%
      }}%
    }%
   \vphantom{\rightarrow}%
  }%
}

\makeatletter
\newcommand{\xrightpitch}[2][]{%
  \ext@arrow 0359\xrightpitchfill@{#1}{#2}%
}
\newcommand{\xrightpitchfill@}{%
  \arrowfill@\relbar\relbar\rightpitch
}
\newcommand{\xrightarrowtriangle}[2][]{%
  \ext@arrow 0359\xrightarrowtrianglefill@{#1}{#2}%
}
\newcommand{\xrightarrowtrianglefill@}{%
  \arrowfill@\relbar\relbar{\mathrel{\smash{\rightarrowtriangle}\vphantom{\rightarrow}}}%
}
\makeatother

\begin{document}
$a\rightpitch b\rightarrowtriangle c$

$a\xrightpitch{\tau\tau'}b\xrightarrowtriangle[\equiv'_n]{\tau\tau'}c$

$a_{\xrightpitch{\tau\tau'}}$
\end{document}

Thanks to Herbert Voß's “Math mode” for the x macros. I smashed \rightarrowtriangle because it's too high for the purpose at hand.

enter image description here

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This is a solution that uses only the MnSymbol font and the nccmath package. I defined \longlpitchfork and longrtrianglearrow commands , that have the same length as \longrightarrow (in the MinionPro version!):

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[mathlf, minionint]{MinionPro}

\usepackage{mathtools}
\usepackage{nccmath}

\renewcommand\dint{\displaystyle\int}
\newcommand\mint{\medint\int}
\newcommand\longlpitchfork{\mathrel{\relbar\mkern-3.15mu\leftpitchfork}}%\relbar\mkern-3.15mu
\newcommand\longrtrianglearrow{\mathref{\relbar\mkern-8.5mu\relbar\mkern-9.3mu\medmath{\smalltriangleright}}}

\begin{document}

\[ \begin{array}{l}
       \overset{τ τ '}{\longlpitchfork}\\
       \overset{τ τ '}{\longrightarrow}  \\
       \overset{τ τ '}{\longrtrianglearrow}
   \end{array} \]

\end{document} 

enter image description here

Finally, here are extensible arrows that use only the MnSymbol font and macros described by Herbert Voss in Math Mode, § 38:

    \documentclass[12pt]{article}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage[mathlf, minionint]{MinionPro}
    \makeatletter

    \newcommand{\xrightfork}[2][]{%
      \ext@arrow 0079\xrightforkfill@{#1}{#2}%
    }
    \newcommand{\xrightforkfill@}{%
      \arrowfill@\relbar\relbar{\mathrel{\smash{\leftpitchfork}}}
    }

    \newcommand{\righttrianglearrow}{%
      \mathrel{% it's a relation
    \relbar\mkern-5mu\smalltriangleright%
      }%
    }
    \newcommand{\xrighttrianglearrow}[2][]{%
      \ext@arrow 0079\xrighttrianglearrowfill@{#1}{#2}%
    }
    \newcommand{\xrighttrianglearrowfill@}{%
      \arrowfill@\relbar\relbar\righttrianglearrow
    }

    \makeatother

    \begin{document}

    \[ \begin{array}{l@{\qquad}l}
     \xrightfork[\equiv ']{\text{either }τ \text{ or else }τ '} & \xrighttrianglearrow[\equiv ']{\text{either }τ \text{ or else }τ '}\\[2ex]
     \xrightfork[\equiv ']{τ \text{ or }τ '} & \xrighttrianglearrow[\equiv ']{τ \text{ or }τ '}
       \end{array} \]

    \end{document} 

And the result of a test file:

enter image description here

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