Typeset a 'not equal' symbol, whose slash is reversed (from the upper-left to the bottom-right)

Question

The not equal symbol (≠), has its slash from the upper-right to the bottom-left. For example:

\begin{equation}
    i  \neq  j
\end{equation}

Typesets the following:

i ≠ j

---

How can I obtain a similar symbol, whose slash directionality is reversed (i.e., from the upper-left to the bottom-right, as displayed below)?

Answer 1

You could make use of \reflectbox (as has been pointed out in the comments), but it won't be usable in super- or subscripts:

\documentclass{article}
\usepackage{graphicx}
\newcommand{\flipneq}{\mathrel{\reflectbox{$\neq$}}}
\begin{document}
$A \flipneq B \neq C$
\end{document}

Answer 2

You can construct this in a similar manner to the methods described in a related question.

For example, adapting a prior answer and enhanced via another, you can do the following:

\documentclass{article}
\usepackage{amssymb}
\usepackage{amsmath}
\makeatletter
\newcommand{\rneq}{\mathrel{\vphantom{\neq}\mathpalette\do@rneq\relax}}
\newcommand{\do@rneq}[2]{%
  \ooalign{%
    $#1\m@th=$\cr
    \hidewidth$#1\m@th\backslash$\hidewidth\cr
  }%
}
\makeatother
\begin{document}
    \Huge
\begin{align*}
    i   &   \neq j\\
    \\
    i   &   \rneq j\\
    \\
    \\
    i   &   \neq_{a} j\\
    \\
    i   &   \rneq_{a} j\\
\end{align*}

\end{document}

This yields:

This has the advantage of working fine with most kerning and sub/super-scripting (as shown), and allowing easy adjustment, by changing the spacing within the \rneq command.

You also can view this example on Overleaf.

Answer 3

You can define a \flipnot symbol that can be used like \not:

\documentclass{article}
\usepackage{amsmath,graphicx}
\makeatletter
\newcommand{\flipnot}{\mathrel{\mathpalette\flipnot@\relax}}
\newcommand{\flipnot@}[2]{%
  \begingroup
  \sbox\z@{$#1\not$}%
  \raisebox{\dimexpr\depth-\dp\z@}{\scalebox{1}[-1]{$\m@th#1\not$}}%
  \endgroup
}
\makeatother
\newcommand{\fneq}{\flipnot=} % \neq is \not=
\begin{document}
$\neq\fneq$
$a\neq b\fneq c^{\fneq}$
\end{document}