Convergence of selected dual-space algorithms on a two-dimensional example. (a) Two convex constraint sets with intersection. (b) Two non-convex constraint sets with several intersections. (c) Two non-convex constraint sets without intersection (infeasible problem). All iterations start from the same point in the right part of the plots. Symbols represent the actual iterates, the dotted lines are connecting consecutive iterates. ER = error–reduction algorithm (12), CFA = charge-flipping algorithm (13), AAR = averaged alternating reflections (20), RAAR = relaxed averaged alternating reflections (23), DM = difference map [(19), with , ].