What symbol for primitive recursive cutoff minus?

Question

I am using $\overset{.}{-}$ for the primitive recursive function between natural numbers such that $m\overset{.}{-}n$ is m minus n if m>=n and otherwise 0. Is there a better and more canonical alternative for a mathematical text?

Answer 1

A stack is an option. The vertical gap between the . and - is set with the argument that is currently {-1pt}.

\documentclass{article}
\usepackage{amsmath,stackengine}
\def\prf{\ensurestackMath{\mathbin{\stackengine{-1pt}{-}{.}{O}{c}{F}{F}{S}}}}
\begin{document}
$m \prf n$ is $m$ minus $n$ if $m\geq n$ and otherwise $0$.
\end{document}

Answer 2

Only a few math fonts provide the symbol and importing it from them might clash with the other symbols.

You can build it yourself:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\prc}{\mathbin{\mathpalette\prc@inner\relax}}
\newcommand{\prc@inner}[2]{%
  \vbox{\offinterlineskip\m@th
    \ialign{%
      ##\cr
      \hidewidth\raisebox{-1.5\height}[0pt][0pt]{$#1.$}\hidewidth\cr
      $#1-$\cr
    }%
  }%
}
\makeatother

\begin{document}

\[
x\prc y-\sum_{i=a\prc b}^{n}a_i
\]

\end{document}

As you see, it scales well in subscripts. Modify the 1.5 factor if the distance does not suit you.

A possible improvement, suggested by Barbara Beeton, is to compare the symbol with \div. The dot would be slightly smaller, but we can scale it.

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

\makeatletter
\newcommand{\prc}{\mathbin{\mathpalette\prc@inner\relax}}
\newcommand{\prc@inner}[2]{%
  \vbox{\offinterlineskip\m@th
    \halign{%
      ##\cr
      \hidewidth
      \raisebox{-1.45\height}[0pt][0pt]{\scalebox{1.2}{$#1.$}}%
      \hidewidth\cr
      $#1-$\cr
    }%
  }%
}
\makeatother

\begin{document}

\[
x\prc y-\sum_{i=a\prc b}^{n}a_i \quad {\prc}{\div}
\]

\end{document}

Answer 3

\documentclass{article}
\usepackage{MnSymbol}
\begin{document}
$x\dotminus y$
\end{document}

As Barbara points out in the comments, MnSymbol replaces all symbols, which has the advantage that the symbols will be similar in style. If you want to access just this single symbol, you have to extract the relevant parts of MnSymbol.sty.

\documentclass{article}
% http://tex.stackexchange.com/a/36088
\DeclareFontFamily{U}{MnSymbolC}{}
\DeclareFontShape{U}{MnSymbolC}{m}{n}{
    <-6>  MnSymbolC5
   <6-7>  MnSymbolC6
   <7-8>  MnSymbolC7
   <8-9>  MnSymbolC8
   <9-10> MnSymbolC9
  <10-12> MnSymbolC10
  <12->   MnSymbolC12}{}
\DeclareSymbolFont{MnSyC}{U}{MnSymbolC}{m}{n}
\DeclareMathSymbol{\dotminus}{\mathbin}{MnSyC}{24}
\begin{document}
$x\dotminus y$
\end{document}

Answer 4

\documentclass{article}
\usepackage{mathabx}
\begin{document}
$x\dotdiv y$
\end{document}

As Enrico points out in the comments, mathabx replaces all symbols, which has the advantage that the symbols will be similar in style. If you want to access just this single symbol, you have to extract the relevant parts of mathabx.sty.

\documentclass{article}
\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{
      <5> <6> <7> <8> <9> <10> gen * mathb
      <10.95> mathb10 <12> <14.4> <17.28> <20.74> <24.88> mathb12
      }{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}
\DeclareMathSymbol{\dotdiv}{2}{mathb}{"01}
\begin{document}
$x\dotdiv y$

\end{document}