The following two figures refer to the radix-2 FFT algorithm’s recursive tree. The first one depicts the recursive calls done in each stage, whereas the second pictorially outlines the intuition behind the bit-reversal permutation.
\documentclass{standalone}
\usepackage[edges]{forest}
\begin{document}
\begin{forest}
for tree={
draw,
fill=gray!30,
s sep+=3ex,
l sep+=4ex,
math content,
edge={semithick},
where n children={0}{}{label={}}}
[{
a_{0}~~
a_{1}~~
a_{2}~~
a_{3}~~
a_{4}~~
a_{5}~~
a_{6}~~
a_{7}
}
[{
a_{0}~~
a_{2}~~
a_{4}~~
a_{6}
}
[{
a_{0}~~
a_{4}
}
[a_{0}]
[a_{4}]
]
[{
a_{2}~~
a_{6}
}
[a_{2}]
[a_{6}]
]
]
[{
a_{1}~~
a_{3}~~
a_{5}~~
a_{7}
}
[{
a_{1}~~
a_{5}
}
[a_{1}]
[a_{5}]
]
[{
a_{3}~~
a_{7}
}
[a_{3}]
[a_{7}]
]
]
];
\end{forest}
\end{document}
\documentclass{standalone}
\usepackage[edges]{forest}
\usepackage{xcolor}
\definecolor{mycolor0}{RGB}{44, 117, 226}
\definecolor{mycolor1}{RGB}{255, 103, 15}
\begin{document}
\begin{forest}
for tree={
draw,
fill=gray!30,
s sep+=1ex,
l sep+=4ex,
math content,
edge={semithick},
where n children={0}{}{label={}}}
[{
a_{00{\color{mycolor0}0}}~~
a_{00{\color{mycolor1}1}}~~
a_{01{\color{mycolor0}0}}~~
a_{01{\color{mycolor1}1}}~~
a_{10{\color{mycolor0}0}}~~
a_{10{\color{mycolor1}1}}~~
a_{11{\color{mycolor0}0}}~~
a_{11{\color{mycolor1}1}}
}
[{
a_{0{\color{mycolor0}0}0}~~
a_{0{\color{mycolor1}1}0}~~
a_{1{\color{mycolor0}0}0}~~
a_{1{\color{mycolor1}1}0}
}, for tree={edge=mycolor0}
[{
a_{{\color{mycolor0}0}00}~~
a_{{\color{mycolor1}1}00}
}
[a_{000}, for tree={edge=mycolor0}]
[a_{100}, for tree={edge=mycolor1}]
]
[{
a_{{\color{mycolor0}0}10}~~
a_{{\color{mycolor1}1}10}
}, for tree={edge=mycolor1}
[a_{010}, for tree={edge=mycolor0}]
[a_{110}, for tree={edge=mycolor1}]
]
]
[{
a_{0{\color{mycolor0}0}1}~~
a_{0{\color{mycolor1}1}1}~~
a_{1{\color{mycolor0}0}1}~~
a_{1{\color{mycolor1}1}1}
}, for tree={edge=mycolor1}
[{
a_{{\color{mycolor0}0}01}~~
a_{{\color{mycolor1}1}01}
}, for tree={edge=mycolor0}
[a_{001}, for tree={edge=mycolor0}]
[a_{101}, for tree={edge=mycolor1}]
]
[{
a_{{\color{mycolor0}0}11}~~
a_{{\color{mycolor1}1}11}
}, for tree={edge=mycolor1}
[a_{011}, for tree={edge=mycolor0}]
[a_{111}, for tree={edge=mycolor1}]
]
]
];
\end{forest}
\end{document}