added sudoku figure

book.tex

@@ -2143,6 +2143,20 @@ board and its corresponding graph. --Jeremy}
 \end{itemize}
 If you can color the remaining nodes in the graph with the nine
 colors, then you have also solved the corresponding game of Sudoku.
+Figure~\ref{fig:sudoku-graph} shows an initial Sudoku game board and
+the corresponding graph with colored vertices.
+
+\begin{figure}[tbp]
+\includegraphics[width=0.45\textwidth]{sudoku}
+\includegraphics[width=0.5\textwidth]{sudoku-graph}
+\caption{A Sudoku game board and the corresponding colored graph.  We
+  map the Sudoku number 1 to blue, 2 to yellow, and 3 to red.  We only
+  show edges for a sampling of the vertices (those that are colored)
+  because showing edges for all of the vertices would make the graph
+  unreadable.}
+\label{fig:sudoku-graph}
+\end{figure}
+
 
 Given that Sudoku is graph coloring, one can use Sudoku strategies to
 come up with an algorithm for allocating registers. For example, one