%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-33 on page 502.
%% 
%%  Copyright (C) 2007 Michel Goossens, Frank Mittelbach, Denis Roegel, Sebastian Rahtz, Herbert Vo\ss 
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{255.83385pt}
\StartShownPreambleCommands
\usepackage[all,arc,dvips]{xy}
\StopShownPreambleCommands
\begin{document}
\[\begin{xy} 0;/r20mm/:
   (0,0)="o" ,*@{*} ,*+!U{o}
  ,(-1.0,0.5)="p"*@{o},*+!LD{p_1}
  ,{\ar @{-->} "o";"p"};
  (-2.,0.)="c",*@{*},*+!L{c_1}
  ,"c",{\ellipse_{}},{\ellipse^{.}}
  ,(1.8,0.4)="p",*@{o},*+!D{p_2}
  ,{\ar @{-->} "o";"p"};
  (0.8,-1)="c",*@{*},*+!R{c_2}
  ,"c",{\ellipse{}}
\end{xy}\]
\end{document}