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I have two variables: \vec{x} and another one which is somehow dual to the first. Therefore I would like to symbolize the second with something similar but a leftarrow on top of the x. I already tried \stackrel{leftarrow}{x} but it looks to different (too big gap between x and the arrow, and too big arrow).
Is there a way to define a \cev command which does what I want?
This may not be the best way, but two \reflectbox commands will do the trick:
\documentclass{article}
\usepackage{graphicx}
\newcommand{\cev}[1]{\reflectbox{\ensuremath{\vec{\reflectbox{\ensuremath{#1}}}}}}
\begin{document}
\[
\vec{a} \quad \cev{a} \quad \vec{b} \quad \cev{b}
\]
\end{document}
t is marked using \cev{t_{\mathring{r}}} and s is marked using \cev{s}_{\mathring{r}} I'd like to have the arrow centered over the whole symbol (including subscript) as it is in the case of t on the left.
t on the left does not appear to be centred over the whole symbol including the superscript to me. On the other hand, \cev{t_{\mathring{r}}} seems to give exactly what you want.
May 26, 2014 at 11:25
\cev{t_{\mathring{r}}} is shown on the right. And \cev{t_r} is shown on the left, where the arrow is a bit more centered than in the case of \cev{t}_r. I'm happy with the looks on the left side, but the right side obviously is wrong.
\cev{t_{\mathring{r}}} on my machine looks nothing like yours! Please post a new question with a complete example code that reproduces the problem.
May 27, 2014 at 7:41
A \cev command that seems to give good results on most letters and works correctly (apart a very small drift) in subscripts and superscripts.
\documentclass{article}
\usepackage{graphicx,accents}
\makeatletter
\DeclareRobustCommand{\cev}[1]{%
\mathpalette\do@cev{#1}%
}
\newcommand{\do@cev}[2]{%
\fix@cev{#1}{+}%
\reflectbox{$\m@th#1\vec{\reflectbox{$\fix@cev{#1}{-}\m@th#1#2\fix@cev{#1}{+}$}}$}%
\fix@cev{#1}{-}%
}
\newcommand{\fix@cev}[2]{%
\ifx#1\displaystyle
\mkern#23mu
\else
\ifx#1\textstyle
\mkern#23mu
\else
\ifx#1\scriptstyle
\mkern#22mu
\else
\mkern#22mu
\fi
\fi
\fi
}
\makeatother
\begin{document}
$Xx$
$X\cev{x}_{\cev{x}_{\cev{x}}}$ $\cev{a}\cev{b}\cev{m}\cev{X}$
$X\vec{x}_{\vec{x}_{\vec{x}}}$ $\vec{a}\vec{b}\vec{m}\vec{X}$
$\cev{\imath}$
$\vec{\imath}$
\end{document}
A different implementation:
\documentclass{article}
\usepackage{graphicx}
\makeatletter
\DeclareRobustCommand{\cev}[1]{%
{\mathpalette\do@cev{#1}}%
}
\newcommand{\do@cev}[2]{%
\vbox{\offinterlineskip
\sbox\z@{$\m@th#1 x$}%
\ialign{##\cr
\hidewidth\reflectbox{$\m@th#1\vec{}\mkern4mu$}\hidewidth\cr
\noalign{\kern-\ht\z@}
$\m@th#1#2$\cr
}%
}%
}
\makeatother
\begin{document}
$Xx$
$X\cev{x}_{\cev{x}_{\cev{x}}}$ $\cev{a}\cev{b}\cev{m}\cev{X}$
$X\vec{x}_{\vec{x}_{\vec{x}}}$ $\vec{a}\vec{b}\vec{m}\vec{X}$
$\cev{\imath}$
$\vec{\imath}$
$\cev{\sigma}\cev{x}$
$\vec{\sigma}\vec{x}$
\end{document}
\cev arrow slants to the left, while the \vec arrow (as well as the italic letter) slants to the right.
May 17, 2015 at 0:21
\cev{x}
\mkern.
The extensible \overleftarrow might do the trick. The problem is that the arrow is bigger than the one used for \vec, as shown by \[\overleftarrow{a}\overrightarrow{a}\vec{a}\] . To have consistent arrows, you should redefine $\vec$ :
\documentclass{minimal}
\renewcommand\vec[1]{\overrightarrow{#1}}
\newcommand\cev[1]{\overleftarrow{#1}}
\begin{document}
\[\cev{a}\vec{a}\]
\end{document}
Edited to add:
Another solution is the \overset command from amsmath, used with \leftarrow. However, the arrow is still bigger than the one from the \vec
command.
\documentclass{minimal}
\usepackage{amsmath}
\begin{document}
\[\overset{\leftarrow}{v}\overset{\rightarrow}{v}\vec{v}\]
\end{document}
*Edited to add *
Using \shortleftarrow from the stmaryrd package slightly improves the above construction.
The \vec is an accent (and its own symbol; hence all \leftarrow and similar constructs look different), and the default Computer Modern font doesn't have a symbol which would be its mirror.
In addition to rotating/mirroring the vec accent symbol, as shown by Ian, you could make use of one provided by, say, STIX/XITS (if you use XeTeX). Then you could define:
\def\cev{\XeTeXmathaccent"0"1"20D6}
% The first number ("0) denotes the math type (0=Ord,1=Op,2=Bin,3=Rel,etc.)
% The second number ("1) denotes the math family (0=Roman,1=Italic,etc.)
% The last number ("20D6) denotes the actual glyph slot
$\cev a \qquad \vec a$
Which could look like: 
I have a little to add to Frederic's solution:
\usepackage{stmaryrd}
\overset{\shortleftarrow}{a} \quad \vec{a} \quad \overset{{}_{\shortleftarrow}}{a}
Basically, the arrow in the subscript makes it smaller and reduces the vertical space between it and the variable. Unfortunately, nesting doesn't seem to work.
Eberhard Mattes provided in the 90s an extra font called emsy containing the missing arrow. The file is downloadable here
https://www.pcorner.com/list/EMTEX
Anyway, it's not easy to use since you have to put the .mf and .tfm files in the correct directory. Once this is done, you may define
\newcommand{\cev}[1]{\mathaccent"7A00{\mkern1mu{#1}}}
