Math letter's italic correction

Question

Are there a way to display PS and TN and CD and so forth (product of two variables) more beautifully? The normal CM font puts a wide space between the left letter and right letter.

If use the package txfonts the $S_1$ turned not so good-looking.

Answer 1

I've nothing to do with txfonts (more exactly, txmi family). Maybe you can use another Times like font package. mathptmx works fine (but it has fewer symbols than txfonts), and if you use XeTeX, you can also use XITS Math for math fonts. Commercial fonts like MathTimePro(2) are also good-looking.

I suggest XITS Math with XeLaTeX:

\documentclass{article}
\usepackage{unicode-math}
\setmainfont{XITS}
\setmathfont{XITS Math}
\begin{document}
See $S_1$, $S_2$, $S_3$.
\end{document}

Answer 2

Leo Liu has answered about subscripts: unfortunately, the tx fonts have some design flaws.

Regarding the space between letters in math mode, that's a design decision by Knuth. The simple juxtaposition of two letters means usually the product of two variables (or parameters). If you need "multiletter identifiers", then use

$\mathit{AB}\mathit{CD}$.

Answer 3

As egreg explained, the space between the letters is a design decision. In most cases I'd stick with TeX's default spacing, but it's possible to remove the italic correction. The result is shown in the second line of the following image.

As you can see, the result is not always desirable: for the combination PW, the letters are much too close to each other. But for WA and Tf, the output looks better with the italic correction removed. Here's my implementation of \ric (remove italic correction):

\documentclass{article}
\newcommand*\ric[1]{\vphantom{#1}\smash{#1_{}\kern-\scriptspace}}
\begin{document}
$PS\ TN\ CD\ WA\ Tf\ PW$

$\ric{P}S\ \ric{T}N\ \ric{C}D\ \ric{W}A\ \ric{T}f\ \ric{P}W$
\end{document}

A short explanation of the code: the italic correction is not used when the character is followed by a subscript, but no superscript. However, a \scriptspace is automatically added to the width of the subscript. I used an empty subscript and a negative kern to compensate the \scriptspace. The rest of the code takes care of the unwanted additional depth caused by the empty subscript.