[main.tex] Add ref to node screening.

main.tex

@@ -1382,7 +1382,7 @@ Solvable by proximal algorithms (ISTA-like, Chambolle-Pock, Condat-Vu, ...), coo
 		\includegraphics[width=.48\linewidth]{images/pruning_example_bad_color.pdf}\\
 		$\npruned$ & $\nunpruned$
 	\end{tabular}
-	\caption{Early pruning illustration: primal and dual iterates.
+	\caption{Early pruning\footnote[frame]{principle close to node screening of \cite{Guyard_node_screening}} illustration: primal and dual iterates.
 % for the optimization of~\eqref{pb:l2pl1_M:node}.
 % (dashed and full lines, respectively), and the best known upper bound (dotted horizontal line).  Left: pruning of the node is achieved after convergence of the primal descent algorithm minimizing~\eqref{pb:l2pl1_M:node} (here in 100 iterations), but the dual value after 10 iterations informs us that the node can be pruned (green circle). Right: in this case, the lower bound $\lbn$ is too low and the node cannot be pruned.
 }