Controling underline depth in math mode

Question

I use the ulem package to control the depth of underlines by setting

\renewcommand{\ULdepth}{1pt}

This works fine outside math mode. However, when using \uline inside math mode, I seem to lose control over the underline depth. (Adding \smash doesn't really solve the problem either.)

\documentclass{article}

\usepackage[normalem]{ulem}
\setlength{\parskip}{15pt} % add space between paragraphs for illustration

\begin{document}

\renewcommand{\ULdepth}{1pt} 
\uline{normal text} $\uline{f(x)=y}$

\uline{normal text} $\uline{\smash{f(x)=y}}$

\renewcommand{\ULdepth}{10pt}
\uline{normal text} $\uline{f(x)=y}$

\end{document}

Is there a way to "regain control" over underline depth within math mode. If such control is not possible with the ulem package (I also tried with soul, which didn't work), are there other packages that can achieve the purpose of making the depth of underlines uniform both inside and outside math mode?

Answer 1

You can use $\mbox{\uline{$....$}}$ if you want to underline a subterm of the math with ulem text settings.

Answer 2

This is an extension to David's answer:

• It defines \mathuline that can be used in math mode to get the text mode version of \uline, using David's trick. Additionally \mathpalette is used to get the correct math style.
• [Optional] \uline is redefined to use \mathuline in math mode instead of the default \underline. But beware, if you have larger symbols and formulas with larger depth (e.g. \int, \frac), then the line will cross the formula.

Example file:

\documentclass{article}
\usepackage{ulem}

\makeatletter
\newcommand*{\saved@uline}{}
\let\saved@uline\uline

\newcommand*{\mathuline}{%
  \mathpalette{\math@uline\saved@uline}%
}
\newcommand*{\math@uline}[3]{%
  % #1: ulem command
  % #2: math style
  % #3: contents
  \mbox{#1{$#2#3\m@th$}}%
}

% optional
\renewcommand*{\uline}{%
  \relax  
  \ifmmode
    \expandafter\mathuline
  \else
    \expandafter\saved@uline
  \fi
}
\makeatother

\begin{document}
\uline{normal text} $\uline{f(x)=y}$
\end{document}