9

In Mac Lane's "Categories for the Working Mathematician" I came across an arrow accented above with a dot, indicating a natural transformation. I failed to find an existing symbol for this. It looks something like

$$\overset{\bullet}{\longrightarrow}$$

but the dot is smaller and close to the arrow. I want to avoid if possible constructing a custom symbol. Does something like this exist? DeteXify could not find one.

4
  • 1
    Have you checked through all the methods listed in How to look up a symbol or identify a math alphabet? What about the Comprehensive LaTeX Symbol List?
    – Werner
    Commented Oct 27, 2013 at 16:42
  • 4
    \xrightarrow{.} (requires amsmath).
    – egreg
    Commented Oct 27, 2013 at 16:44
  • @Werner: Before posting I tried google, Detexify, and (the arrows section of) the comprehensive list. At Mico: Your solution (with .75 ex instead of 1 ex) is quite satisfactory.
    – alex
    Commented Oct 27, 2013 at 17:11
  • so there is no unicode symbol for that guy ?
    – nicolas
    Commented Jan 15, 2016 at 8:23

2 Answers 2

5

This is similar to Mico's, but doesn't use \ooalign, since an approach without overlapping rules seems better:

\documentclass{article}
\usepackage{amsmath}
\usepackage{graphicx}
\newcommand{\naturalto}{%
  \mathrel{\vbox{\offinterlineskip
    \mathsurround=0pt
    \ialign{\hfil##\hfil\cr
      \normalfont\scalebox{1.2}{.}\cr
%      \noalign{\kern-.05ex}
      $\longrightarrow$\cr}
  }}%
}
\begin{document}
$F\naturalto G\overset{\text{\normalsize.}}\longrightarrow H$
\end{document}

This won't scale in sub/superscripts, but I can't see this as a real limitation. The commented line shows where to act for raising or lowering the dot. The second arrow is just for comparison.

enter image description here

2
3

Revised suggestion (after receiving quite a few more details and suggestions from @egreg, and after finding a copy of "Categories for the Working Mathematician" online):

enter image description here

\documentclass{article}
\usepackage{graphicx,newtxtext,newtxmath}
\newcommand{\dotarrow}{% to be used in math mode...
   \mathrel{\ooalign{\hss\raise.65ex\hbox{\scalebox{1.25}{\normalfont .}}%
   \kern0.35ex\hss\cr$\longrightarrow$}}}
\begin{document}
$\tau^{-1}: T \dotarrow S$ 
\end{document}

Comment: On one and the same page (p. 24) of the book, there are at least two non-identical versions of the "arrow with large dot on top" symbol. (This may be due, in part, to the low resolution scan of the book I've found...) The form shown above has the dot resting on top of the arrow. In another version, the dot floats slightly above the arrow. If you prefer that look, use \raise0.75ex instead of \raise0.65ex when determining the dot's vertical position.

6
  • No, a simple period is used.
    – egreg
    Commented Oct 27, 2013 at 16:46
  • @egreg - OK. Thanks. (I don't have access to the publication "Categories for the Working Mathematician"...)
    – Mico
    Commented Oct 27, 2013 at 16:47
  • Actually I was wrong. I have the Italian translation, where most of the displayed formulas are taken from the original (and the font doesn't match). In inline formulas a period is used, in displayed ones there's a tiny bullet, much smaller than yours (but distinguishable from a period) and lowered.
    – egreg
    Commented Oct 27, 2013 at 16:53
  • @egreg - I just gave it another try.
    – Mico
    Commented Oct 27, 2013 at 17:05
  • It's lower than that and perhaps a bit fatter (\scalebox{1.2} might be). Rather than \ooalign I'd use \vbox{\offinterlineskip\ialign{\hfil##\hfil\cr\scalebox{1.2}{\normalfont.}\cr\noalign{\kern-.1ex}$\longrightarrow$\cr}}
    – egreg
    Commented Oct 27, 2013 at 17:15

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .