in math-mode, underline adjacent variables, except with space between underlines

Question

In mathmode, is there a way to underline a veriable that doesn't make the underline join to the next variable? Example:

\underline{X}\underline{Y}

except I want a break in the underline between X and Y.

the way i'm doing it now is comersome:

\text{ }\underline{X}\text{ }\underline{Y}

would be nice if there were a way to get an underline that is shorter and centers on the character so i doesn't bleed to the edge of the next character. because I'm trying to write matrix equations that have many vector variables marked with underlines...

Answer 1

Underlining is ugly. You can somehow mitigate it by shortening the underline:

\documentclass{article}
\usepackage{amsmath}

\renewcommand{\vec}[1]{%
  \mspace{2mu}%
  \underline{\mspace{-2mu}#1\mspace{-2mu}}%
  \mspace{2mu}%
}

\begin{document}

$\alpha\vec{X}\vec{Y}$

$\alpha XY$

\end{document}

The second line is for comparison.

Another strategy could be using accents:

\documentclass{article}
\usepackage{amsmath,accents}

\renewcommand{\vec}[1]{\underaccent{\bar}{#1}}

\begin{document}

$\alpha\vec{X}\vec{Y}$

$\alpha XY$

\end{document}

My preference would go to \bm:

\documentclass{article}
\usepackage{amsmath,bm}

\renewcommand{\vec}[1]{\bm{#1}}

\begin{document}

$\alpha\vec{X}\vec{Y}$

$\alpha XY$

\end{document}

In any case use proper markup. You might not want to redefine \vec and use a different command name. This way, you can change the appearance of all vectors by just changing the definition.