Change `\textlnot` to something usable in math mode with `kpfonts`

Question

Consider the following MWE:

\documentclass{article}

\usepackage{kpfonts}
\usepackage[utf8]{inputenc}

\begin{document}

This is a formula: $¬¬p \implies p$

\end{document}

The output does not typeset the ¬ unicode symbol (NOT SIGN) and I get this warning:

LaTeX Warning: Command \textlnot invalid in math mode on input line 8

It seems that kpfonts is taking care of handling ¬ but it assigns to it a command that works in text mode (which seems a strange default since this is a mathematical symbol).

How can I redefine the symbol to something usable in math mode (e.g. \lnot) or, better, to make it expand to \textlnot in text mode and \lnot in math mode?

Answer 1

The definition of ¬ as \textlnot is done by inputenc with the utf8 option, because kpfonts loads textcomp and ts1enc.dfu has

\DeclareUnicodeCharacter{00AC}{\textlnot}

You can load newunicodechar and fix it.

\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{kpfonts}
\usepackage{newunicodechar}

\newunicodechar{¬}{\TextOrMath{\textlnot}{\lnot}}

\begin{document}

This is a formula: $¬¬p \implies p$

\end{document}

Answer 2

Something like this (making it active)?

\documentclass{article}

\usepackage{kpfonts}
\usepackage[utf8]{inputenc}

\catcode"00AC=\active
\def¬{\ifmmode\lnot\else\textlnot\fi}

\begin{document}

This is a formula: $¬¬p \implies p$

\end{document}