Spacing in variable names in Math mode too large

Question

I have some variables with multiple letter names like UA, US or ET. When I write the variables starting with an 'U' in math mode it seems like the space afterward the U is too large (look at the space between 'U' and 'A' below). Especially if I have other characters before the 'U' like a capital Delta:

\documentclass[11pt]{article}
\usepackage{lmodern}
\begin{document}
$US_t + \Delta UA_t - \Delta ET_t $
\end{document}

Answer 1

I'd say that the general rule is if a var name is more than one letter, then it has to be written upright to avoid being confused with a product. I'd use something like \newcommand\US{\mathrm{US}} etc and then use \US. Then if you'd rather like \mathit as Christian suggests, then you can easily change that in a single place.

I've also included Christians suggestion of using \tlvar. I tend to prefer making direct macros as they are faster to type, and you are more free to define each like you want them. For example if you use a macro that ought to need some adjustments if a subscript is used on this particular macro, then you can use xparse to redefine this particular macro to do something extra.

\documentclass[a4paper]{article}

\newcommand\US{\mathrm{US}}
\newcommand\UA{\mathrm{UA}}
\newcommand\ET{\mathrm{ET}}
\newcommand\tlvar[1]{\mathrm{#1}}

\begin{document}
Version 1: $US_t + \Delta UA_t - \Delta ET_t $

Version 2: $\US_t + \Delta \UA_t - \Delta \ET_t $

Version 3: $\tlvar{US}_t + \Delta \tlvar{UA}_t - \Delta \tlvar{ET}_t $


\end{document}

Answer 2

The essentiel answer on how to avoid that problem is given in the comments. Let's test OP's initial remark:

When I write the variables starting with an 'U' in math mode it seems like the space afterward the U is too large (look at the space between 'U' and 'A' below).

Let's see:

\documentclass[11pt]{article}
\usepackage{lmodern, amsmath}
\setlength{\fboxsep}{0pt}
\begin{document}
$US_t + \Delta UA_t - \Delta ET_t $

$\boxed{U}\boxed{S}_t + \boxed{\Delta} \boxed{U}\boxed{A}_t - \boxed{\Delta}\boxed{E}\boxed{T}_t $

\end{document}

LaTeX does add no extra space after italic U. No font kerning occurs at all, TeX basically puts box after box without considering the content and how well it fits together.

Answer 3

The reason why it looks like there is more space after the U is because the top of the U goes further then the bottom. When u use emphasis the top part of the U will only extend more then when you write it in standard font.