一个基于stackengine
的方案为:
\documentclass{article}
\usepackage{centernot}
\usepackage{stackengine}
\usepackage{amsmath,amssymb}
\newcommand*{\notLongrightarrow}{\centernot{\Longrightarrow}}
\newcommand*{\notLongleftarrow}{\centernot{\Longleftarrow}}
\newcommand*{\onlyRtoL}[1][-2pt]{\mathrel{\stackanchor[#1]{$\Longleftarrow$}{$\notLongrightarrow$}}}
\begin{document}
\[A \Longrightarrow B \notLongrightarrow C\]
\[A \Longleftarrow B \notLongleftarrow C\]
\[A \mathrel{\stackanchor{$\Longleftarrow$}{$\Longrightarrow$}} B \]
\[A \mathrel{\stackanchor[4pt]{$\notLongleftarrow$}{$\Longrightarrow$}} B \]
\[
\lim_{n\to\infty} y_n \quad \onlyRtoL[5pt] \quad |y_1|+|y_2|+\cdots+|y_n|\leq M\]
\end{document}
使用 tikz 绘制可以精细调节位置,使用 joinbox 较难处理位置,而且对行间公式不方便
\documentclass{article}
\usepackage{tikz,joinbox,centernot}
\newcommand{\noiffa}{%
\mathrel{\raisebox{-1.2ex}{{\hbox{%
\joinbox[sep=0.3ex]{$\Longleftarrow$}{$\centernot\Longrightarrow$}%
}}}}%
}
\newcommand{\noiffb}{%
\tikz[baseline={([yshift=-.3ex]current bounding box.center)}]{
\node (a) {$\Longleftarrow$};
\node at ([shift={(0.1em,-0.6ex)}]a.south) {$\centernot\Longrightarrow$};
}}
\begin{document}
$\sum_{i}^{n}a_i \noiffa ABC, \quad \sum_{i}^{n}a_i \noiffb ABC$
\[
\sum_{i}^{n}a_i \noiffb ABC
\]
\end{document}
也可以用
\atop
,能少打几个字符