如何设置上方带有水平线的矢量和张量符号

Question 1

您可以使用该accents包：

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

\begin{document}

$\accentset{\rightharpoonup}{x}$

\end{document}

Answer

您可以使用该accents包：

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

\begin{document}

$\accentset{\rightharpoonup}{x}$

\end{document}

Question 2

像这样？

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[ascii]{inputenc}

\newcommand*{\ttensor}{\mathbin{\overline{\otimes}}}



\begin{document}
Text \( a\ttensor b \) text.
\end{document}

附录：我仍然欠你一个你实际提出的问题的答案。 Christian 的回答非常好，特别是因为它提供了你似乎要求的可扩展鱼叉（当你说覆盖整个单词时）；但假设你不喜欢该fdsymbol包为此使用的字体，并且你想坚持使用“Computer Modern 鱼叉”（在字体的位置"28、"29、"2A和"2B中找到的那些cmmi）；在该包的帮助下amsmath，“手动”制作所需的命令并不困难：

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[ascii]{inputenc}
\usepackage{amsmath}

\makeatletter

\newcommand*\MY@leftharpoonupfill@{%
    \arrowfill@\leftharpoonup\relbar\relbar
}
\newcommand*\MY@rightharpoonupfill@{%
    \arrowfill@\relbar\relbar\rightharpoonup
}
% These are not needed, for now:
% \newcommand*\MY@leftharpoondownfill@{%
%   \arrowfill@\lefttharpoondown\relbar\relbar
% }
% \newcommand*\MY@rightharpoondownfill@{%
%   \arrowfill@\relbar\relbar\rightharpoondown
% }
\newcommand*\overleftharpoon{%
    \mathpalette{\overarrow@\MY@leftharpoonupfill@}%
}
\newcommand*\overrightharpoon{%
    \mathpalette{\overarrow@\MY@rightharpoonupfill@}%
}

\makeatother



\begin{document}

A formula: \(
    \overrightharpoon{x}:=\overrightharpoon{\textit{counter}}
\).  Another formula: \(
    \overleftharpoon{x}:=\overleftharpoon{\textit{counter}}
\).

\end{document}

输出如下：

第二次补充：再三考虑，我觉得覆盖鱼叉的尺寸可能太大了：如果\scriptstyle尺寸合适的话（当主尺寸为文本尺寸时），效果会更好。这需要多花点功夫：

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not necessary, but recommended.

\usepackage[ascii]{inputenc}     % Just to check that the source is still pure,
                                 % 7-bit-clean ASCII when you execute it, as it
                                 % was when I wrote it.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{amsmath}

\makeatletter

\newcommand*\MY@leftharpoonupfill@{%
    \arrowfill@\leftharpoonup\relbar\relbar
}
\newcommand*\MY@rightharpoonupfill@{%
    \arrowfill@\relbar\relbar\rightharpoonup
}
% These are not needed, for now:
% \newcommand*\MY@leftharpoondownfill@{%
%   \arrowfill@\lefttharpoondown\relbar\relbar
% }
% \newcommand*\MY@rightharpoondownfill@{%
%   \arrowfill@\relbar\relbar\rightharpoondown
% }
\newcommand*\overleftharpoon{%
    \mathpalette{\overarrow@\MY@leftharpoonupfill@}%
}
\newcommand*\overrightharpoon{%
    \mathpalette{\overarrow@\MY@rightharpoonupfill@}%
}

% Support for smaller size arrows:
\newcommand*\@dblsty@mathpalette[2]{%
    % Works like "\mathpalette", but macro passed in #1 must take
    % (at least) three arguments, of which the first _two_ are
    % style selections.
    \mathchoice
        {#1\displaystyle       \scriptstyle       {#2}}%
        {#1\textstyle          \scriptstyle       {#2}}%
        {#1\scriptstyle        \scriptscriptstyle {#2}}%
        {#1\scriptscriptstyle  \scriptscriptstyle {#2}}%
}
\newcommand*\@dblsty@overarrow@[4]{%
    % #1 := stretchable covering arrow
    % #2 := base style
    % #3 := style for covering arrow
    % #4 := base symbol
    \vbox{\ialign{##\crcr
        #1#3\crcr
        \noalign{\nointerlineskip}%
        $\m@th\hfil #2#4\hfil$\crcr
    }}%
}
\newcommand*\smalloverleftharpoon{%
    \@dblsty@mathpalette{\@dblsty@overarrow@\MY@leftharpoonupfill@}%
}
\newcommand*\smalloverrightharpoon{%
    \@dblsty@mathpalette{\@dblsty@overarrow@\MY@rightharpoonupfill@}%
}

\makeatother



\begin{document}

\section{Normal size}

A formula: \(
    \overrightharpoon{x}:=\overrightharpoon{\textit{counter}}
\).  Another formula: \(
    \overleftharpoon{x}:=\overleftharpoon{\textit{counter}}
\).

\section{Smaller size}

A formula: \(
    \smalloverrightharpoon{x}:=\smalloverrightharpoon{\textit{counter}}
\).  Another formula: \(
    \smalloverleftharpoon{x}:=\smalloverleftharpoon{\textit{counter}}
\).

\end{document}

在输出中，您可以比较两种尺寸并选择您喜欢的尺寸。但请注意，较小的尺寸与包中“over-arrows”的其他用法不一致amsmath。

第三次补充： 需要注意的是，这两个命令（的强版本）\overleftharpoon和\overrightharpoon定义在确切地和上面展示的一样，通过包装MnSymbol。另一方面，据我所知，“小”版本是正品新品。

Answer