[texhax] Cases for italic correction

[Khaled Hosny]

On Fri, Jul 20, 2012 at 09:48:50AM -0400, Barbara Beeton wrote:
> On Fri, 20 Jul 2012, Philip TAYLOR wrote:
> 
>     Barbara Beeton wrote:
> 
>     > unlike all other letters in the cm math italic
>     > font, the d has no built-in "overhang" spacing.
>     > i've never actually asked, but i think the
>     > reason for this is that knuth uses d only for
>     > indicating derivatives (as in "dx"), not for
>     > variables, and for that use, he prefers it to
>     > be directly adjacent to the variable involved,
>     > not separated by a small space.
> 
>     But what of a second derivative ?  That would require d^2y/dx^2,
>     which will surely generate almost exactly the same inteference
>     as Michael asserts obtains in d^3 (but which Lars and I cannot see).
> 
> good question.  there's not a single one
> in the texbook -- i checked with grep in
> texbook.tex; i also checked for "d{}" and
> "d\/" and several other increasingly
> obscure possibilities ... nothing.
> 
> so we have no direct knuthian example.
> 
> however, i don't see the difference either;
> here's my test:
> 
>   \documentclass[12pt]{article}
>   \begin{document}
>   \Large
>   $dx + d^2x + d{}^2x + d\/^2x$
>   \end{document}
> 
> definitely a puzzlement.
> 						-- bb

I just checked cmmi10.tfm, there is no italic correction for the d
indeed, so any of those construction wouldn't make any visible
difference.

A better test:

\tracingonline=1
\showboxbreadth=\maxdimen
$d^3 d\/^3 d{}^3$
\showlists
\bye

Which gives:

\teni d
\hbox(4.51111+0.0)x4.48613, shifted -3.62892
.\sevenrm 3
\teni d
\kern0.0
\hbox(4.51111+0.0)x4.48613, shifted -3.62892
.\sevenrm 3
\teni d
\hbox(0.0+0.0)x0.0
\hbox(4.51111+0.0)x4.48613, shifted -3.62892
.\sevenrm 3

So \/ is inserting a zero kern, and {} is inserting a zero width box,
nothing else.

Regards,
 Khaled