DescriptionTits index 2Dn.svg	English: The Tits index of type 2Dn, created using tikz.
Source	“Classification of algebraic semisimple groups”, in Algebraic Groups and Discontinuous Subgroups (Proc. Sympos. Pure Math., Boulder, Colo., 1965), Providence, R.I.: American Mathematical Society, 1966, pages 33–62
Author	David Schwein
SVG development InfoField	The SVG code is valid.   This diagram was created with an unknown SVG tool.

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Source code:

\documentclass{standalone}

\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc,decorations.pathreplacing,decorations.markings}
\usetikzlibrary{fit,math,positioning,arrows.meta,shapes.geometric}
\tikzset{
n/.style={fill=black,circle, %% node
 inner sep=0pt,minimum size=4pt},
e/.style={fill=black,circle, %% ellipses
 inner sep=0pt,minimum size=2pt},
w/.style={fill=white,draw=white,circle, %% white (omitted) nodes
 inner sep=0pt,minimum size=4pt},
c/.style={ellipse,draw=black,inner sep=1pt}, %% circles 
b/.style={decoration={brace,amplitude=10pt},decorate,thick}, %% brace
a/.style={postaction={decorate,decoration={
  markings, mark=at position .5 with 
  {\arrow[black]{Straight Barb[length=\al]}}
 }}} %% mid arrow
} 

\def\v{0.5cm}  %% vertical node space
\def\h{1cm}    %% horizontal node space
\def\s{1.8pt}  %% space for weighted edges
\def\al{5pt}   %% arrow length
\def\l{2.5*\v} %% label separator

\begin{document}
\begin{tikzpicture}

%% Nodes
\node[n] (1) at (0,0) {};

\foreach \x [evaluate=\x as \y using int(\x-1)] in {2,...,18} {
 \node[n] (\x) [right=\h of \y] {};
}

\node[n] at ($(18) + (\h,0.5*\v)$)  (19) {};
\node[n] at ($(18) + (\h,-0.5*\v)$) (19') {};

%% Solid Edges
% single
\foreach \x [evaluate=\x as \y using int(\x+1)] 
in {1,3,4,6,7,11,12,14,15,17} {
 \draw (\x.center) -- (\y.center);
}

\draw[out= 45,in=180] (18.center) to (19.center);
\draw[out=-45,in=180] (18.center) to (19'.center);

%% Ellipses
\foreach \x [evaluate=\x as \y using int(\x+1)]
in {2,5,8,9,10,13,16} {
 \foreach \i in {0.25,0.5,0.75} {
  \node[e] at ($(\x)!\i!(\y)$) {};
 % \draw[white] (\x) -- node [black] {\Large$\cdots$} (\y);
 }
}

%% Omitted Nodes
\foreach \x in {9,10} {
 \node[w] at (\x) {};
}

%% Circled Nodes
\foreach \x in {4,7,12,15} {
 \node[c,fit=(\x)] at (\x) {};
}

%% Braces
% Length 2
\foreach \x [evaluate=\x as \y using int(\x + 1)]
in {5,13} {
 \node (b\x-1) at ($(\x) + (-0.5*\h,-\v)$) {};
 \node (b\x-2) at ($(\y) + (0.5*\h,-\v)$) {};
 \draw [b] (b\x-2) -- node [label={[yshift=-\l] $d-1$}] {} (b\x-1);
}

% Length 3
\node (b1-1) at ($(1) + (-0.5*\h,-\v)$) {};
\node (b1-2) at ($(3) + (0.5*\h,-\v)$) {};
\draw [b] (b1-2) -- node [label={[yshift=-\l] $d-1$}] {} (b1-1);

% Length 4
\node (b16-2) at ($(19') + (0.5*\h,-\v)$) {};
\node (b16-1) at ($(b16-2) - (4*\h,0)$) {};
\draw [b] (b16-2) -- node [label={[yshift=-\l] $n-rd$}] {} (b16-1);

\end{tikzpicture}
\end{document}