Symbol for "not absolutely continuous" in Latex

The \centernot package is made for that:

\documentclass{article}
\usepackage{amsmath,centernot}

\newcommand{\nll}{\centernot{\ll}}

\begin{document}

$a\ll b$

$a\nll b$

\end{document}

txfonts has \nll:

\documentclass{article}

\usepackage{txfonts}

\begin{document}

\[
\nll
\]

\end{document}

MnSymbol also has this letter:

\documentclass{article}


\usepackage{MnSymbol}

\begin{document}

\[
\nll
\]

\end{document}

In the modern toolchain, \not\ll appears to work fine with most Unicode math fonts. You can declare \newcommand\nll{\mathrel{\not\ll}} if you prefer.

\documentclass{article}
\usepackage{unicode-math}

\begin{document}
\( \not\ll \)
\end{document}

As always with unicode-math, compile with LuaLaTeX or XeLaTeX.