%!TEX TS-program = lualatex
\documentclass[12pt]{article}
\usepackage{tikz}
%\usetikzlibrary{fpu}
\usepackage{amsmath, luamaths, luaset}
\usepackage{pgfplots}
\usepgflibrary{luamath}
\pgfplotsset{compat=1.12}
\begin{document}
\begin{tikzpicture}[%
declare function={%
a=3;b=-9;c=8;d=0;
f(\x)=a*\x*\x*\x+b*\x*\x+c*\x+d;
}
]
\pgfkeys{%
/pgf/number format/frac,
/pgf/number format/frac whole=false,
}
\pgfmathparse{f(2/3)}%
$ f\left(\dfrac{2}{3}\right)=
\pgfmathprintnumber\pgfmathresult$
\pgfluamathparse{f(4/3)}%
$ f\left(\dfrac{4}{3}\right)=
\pgfmathprintnumber\pgfmathresult$
\end{tikzpicture}
\mathLog{2}
%\def\x{lfrac(1,2)}
%\def\y{lfrac(-3,4)}
%\def\c{lcomplex(lfrac(3,4),6)}
%\def\d{lcomplex(3,-9)}
%
\(x + y = \mathOp{1 + lfrac(1,2)}\)
\end{document}
