# Command \d invalid in math mode [duplicate]

$$\d{u} \left( t \right) = Z$$


Am I missing a package or what?

• Insert a minimal code to show the error. It works here. Sep 12 '14 at 17:30
• @Sigur Note that the “u” will not be math italic, so “it works here” seems too bold a statement. ;-) Sep 12 '14 at 17:44

## 1 Answer

The fact that a warning is issued should not be underestimated and, indeed, the form of the “u” is wrong, because it's upright and not math italic.

The accents package provides a way to get underaccents.

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\begin{document}
$\underaccent{\dot}{u}(t)=Z$
\end{document}


If you often need the underdot, you can define your own command:

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

\newcommand{\udot}[1]{\underaccent{\dot}{#1}}

\begin{document}
$\udot{u}(t)=Z$
\end{document}


By the way, don't use \left and \right in that context; the do nothing good and something bad. Compare the result with \left and \right added and look carefully at the spacing.

• Isn't there a command \underdot by default? I think I might've seen it. Sep 12 '14 at 17:46