How can I get a dot over a vector?

Question

I would like to combine, somehow, \vec{x} and \dot{x} so that I get an x with a vector arrow over it and a dot over that. Any suggestions?

Answer 1

How about the obvious, i.e. $\dot{\vec{a}}$? Spacing is a little off... which could be fixed by using the esvect package (for vectors in general) and using:

\documentclass{minimal}

\usepackage{esvect}

\begin{document}
$\dot{\vv{a}}$
\end{document}  

Result using first solution is on left hand side, using esvect is on right hand side:

---

UPDATE:
I added a little solution of my own which has good spacing and uses the normal latex vector arrow [but requires the accents package]. This can be obtained using the command \DotVec (no need for an additional \vec; code:

\usepackage{accents}
\newcommand*{\DotVec}[1]{%
  \accentset{\mbox{\hspace{0.25ex}.}}{\ensuremath{\vec{#1}}}}  

I created a document for comparing my solution, the "plain" latex solution and @Eddy_Em's [I hope that's okay?] solution. Code is:

\documentclass{minimal}

%% solution using esvect
\usepackage{esvect}

%% required for looping all letters
\usepackage{pgffor}

%% my solution using the accents package
\usepackage{accents}
\newcommand*{\DotVec}[1]{%
  \accentset{\mbox{\hspace{0.25ex}.}}{\ensuremath{\vec{#1}}}}

%% this is Eddy_Em's solution
\newsavebox{\hght}
\def\dotvec#1{\savebox{\hght}{$\vec{#1}$}\dot{\raisebox{0pt}[.8\ht\hght]{$\vec{#1}$}}}

%% this is used for a little extra vertical space in the below listing
\def\vSpacer{\vrule height 15pt width 0pt}

\begin{document}
\noindent
Order is:
\begin{verbatim*}
\dot{\vec{a}} \dot{\vv{a}} \dotvec{a} \DotVec{a}
\end{verbatim*}
For roman alphabet:\\[5pt]

\par\noindent
\foreach \l in {a,...,z} {%
    \vSpacer $\dot{\vec{\l}} \; \dot{\vv{\l}} \; \dotvec{\l} \; \DotVec{\l},\quad$
}

\par\noindent
\foreach \l in {A,...,Z} {%
    \vSpacer $\dot{\vec{\l}} \; \dot{\vv{\l}} \; \dotvec{\l} \; \DotVec{\l},\quad$
}

\end{document}  

Resulting document:

Zoom for letter a:

Answer 2

I did it so:

\newsavebox{\hght}
\def\dotvec#1{\savebox{\hght}{$\vec{#1}$}\dot{\raisebox{0pt}[.8\ht\hght]{$\vec{#1}$}}}

Some explanations:

• \newsavebox{\boxname} defines a "saving-box" — a box which can contain anything for some preprocessing (for example every page before output is placed to \box255);
• \savebox{\boxname}{text} puts text into box \boxname, we put there our vector;
• then we need to add a dot to it (\dot), but to avoid a large space between dot and vector we "reduce" height of our vector by 20% (\raisebox not only "raise" its contents but also can change its height and depth).

Here we've used \savebox only to measure height of vector, but instead of vector itself in \raisebox argument we can write \usebox{\hght}. It could be more quickly because latex won't need to process this box again — it will simply use a ready box.