Command \d invalid in math mode

Question

\begin{equation}
\d{u} \left( t \right) = Z
\end{equation}

Am I missing a package or what?

Answer 1

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.