In some theories on predication \eta is used instead of set theoretical \in.
How do I get a useful symbol for \noteta which is to \eta as \notin is to \in?
The solutions should be compilable both with pdfLaTeX and LuaLaTeX.
\documentclass{article}
\begin{document}
$\not\eta$
\end{document}
Different math font need different treatment, I'm afraid. But we can use a couple of parameters in order to adjust the positioning for a particular font.
\documentclass{article}
%\usepackage{unicode-math}
%\setmathfont{STIX Two Math}
%\setmathfont{TeX Gyre Pagella Math}
\makeatletter
\DeclareRobustCommand{\noteta}{\mathrel{\mathpalette\noteta@{\noteta@kern\noteta@raise}}}
\newcommand{\noteta@}[2]{\noteta@@#1#2}
\newcommand{\noteta@@}[3]{%
\mkern#2
\ooalign{\hidewidth\raisebox{#3\height}{$#1\m@th/$}\hidewidth\cr$#1\m@th\mkern-#2\eta$\cr}%
}
\newcommand{\noteta@kern}{1mu}% tailor to the font
\newcommand{\noteta@raise}{-0.1}% tailor to the font
\makeatother
\begin{document}
$a\notin A$
$a\noteta A$ $\scriptstyle a\noteta A$
$a\mathrel\eta A$ $\scriptstyle a\mathrel\eta A$
\end{document}
pdflatex
Just loading unicode-math (uncomment line 2)
Uncomment lines 2 and 3
Uncomment lines 2 and 4
This probably needs a different amount of raising/lowering. Changing \newcommand{\noteta@raise}{-0.1} into
\newcommand{\noteta@raise}{0}
results in
Experiment with the math font you use, but do remember that \mathrel\eta is needed if you want to use the symbol as a math relation.
It seems to me that the most reliable approach is to overlay a stroke at the desired angle yourself (while still scaling it properly across different math styles). This can be done in a way that works with both pdfLaTeX and LuaLaTeX. The stroke is drawn using \rule{...}{...}; \notetaangle controls the angle, and \notetashift controls the stroke’s vertical placement. All parameters are under your control; for example:
\documentclass{article}
\usepackage{graphicx} % \rotatebox
\newcommand*\notetaangle{25}
\newcommand*\notetashift{.4ex}
\makeatletter
\newcommand{\noteta}{\mathrel{\mathpalette\noteta@{}}}
\newcommand{\noteta@}[1]{%
\ooalign{%
$#1\eta$\cr
\hidewidth
\raisebox{\notetashift}{%
\rotatebox[origin=c]{\notetaangle}{%
$#1\rule{1.05em}{.10ex}$%
}%
}%
\hidewidth\cr
}%
}
\makeatother
\begin{document}
$\eta\quad\noteta$
\vskip10pt
$x\noteta y$
\end{document}
You could maybe use \mathclap, from the mathtools package
\NewDocumentCommand{\noteta}{}{%
\mathclap{\not}\eta%
}
Example:
\documentclass{article}
\usepackage{mathtools}
\NewDocumentCommand{\noteta}{}{%
\mathclap{\not}\eta%
}
\begin{document}
$x\noteta y$ $\scriptstyle x\noteta y$
\end{document}
\not\eta? ?% harmless under LuaLaTeX, required for pdfLaTeXis referring to, but if you mean the line above, thenT1encoding should (almost)never be used with luatex it breaks all the unicode support, so not exactly harmless, andutf8is not required for pdflatex, unless your latex is 10 years old