What is wrong with the $f$ symbol?

Question

I came across some spacing glitches when I use the f symbol (I'm not having similar weirdness with most other symbols). Here's a very simple MWE to show the issue:

\documentclass[11pt,letterpaper,twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
\usepackage{microtype}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{mathtools}
\usepackage{tensor}
\newcommand*{\kp}[1]{\mskip+#1mu}
\newcommand*{\kn}[1]{\mskip-#1mu}

\begin{document}

    \begin{equation}
        \frac{d f}{d r} = \frac{d\kp1 f}{d\kp1 r} = \frac{d\kp1 \kn2 f}{d\kp1 r}.
    \end{equation}

\end{document}

Preview:

The case A is too tight for my taste. I frequently add some small space between symbols to "kern" my equations. Adding a simple 1/3 of a small space (\,) between d and f gives an output space that is too large, while it's not the case with other symbols (case B). Then adding 1/3 and removing 2/3 gives a nice output (case C). What is going on here? How can I use the f symbol without having to add some kerning each time with it?

Answer 1

The cmmi10 font (and this is followed by lmmi10) has specifically a kerning between d and f:

(CHARACTER C d
   (CHARWD R 0.520488)
   (CHARHT R 0.694445)
   (COMMENT
      (KRN C Y R 0.055555)
      (KRN C Z R -0.055555)
      (KRN C j R -0.111112)
      (KRN C f R -0.166667)
      (KRN O 177 R 0.166672)
      )
   )

exactly in order to avoid the unsightly hole that you seem to like.

Here's my suggestions: I'd use \standarddiff, you might prefer \chamdiff (rename your choice to \diff or whatever you like).

\documentclass{article}
\usepackage{amsmath}
\usepackage{lmodern}
\usepackage{fixcmex} % don't forget with lmodern

\newcommand{\standarddiff}{\mathop{}\!d}
\newcommand{\chamdiff}{\mathop{}\!d\/}

\begin{document}

$\standarddiff f \quad \standarddiff j \quad \standarddiff r$

$\chamdiff f \quad \chamdiff j \quad \chamdiff r$

$d\mkern1mu f$ $d\mkern-1mu f$

\end{document}

Either no kerning (1mu is not good, in my opinion), or the standard.

Answer 2

You might be interested in the esdiff package, which takes care of the spacing problem, and simplifies the typing of Leibniz notation, for both ordinary and partial derivatives. By default the differential symbol is in roman, but you have the [italic] option at loading time.

Unrelated: needless to load amsmath if you load mathtools, which is an extension of the former.

Here is an example:

\documentclass[11pt, letterpaper, twoside]{book}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
\usepackage{microtype}
\usepackage{amsfonts}
\usepackage{mathtools}

\usepackage[italic]{esdiff}

\begin{document}

    \[ \diff{f}{r}, \quad \diff[2]{f}{r}, \qquad \diffp{h}{{x^2}{y}}. \]

\end{document} 

Answer 3

You could to use also diffcoeff package like this minimal example:

\documentclass[a4paper,12pt]{article}
\usepackage{diffcoeff}
\begin{document}
\[\diff[1]fr=\diff{\onemu f}{r}=\diff {\nilmu f}{\onemu r}\]
\end{document}

where the correct spacing are defined from the commands:

\negmu insert spacing of −1 mu

\nilmu insert spacing of 0 mu

\onemu insert spacing of 1 mu

\twomu insert spacing of 2 mu

See the guide to the pag. 7/32.