@article {2020a-NavSabTogPucFra,
	title = {Direct Force Feedback Control and Online Multi-task Optimization for Aerial Manipulators},
	journal = {IEEE Robotics and Automation Letters},
	volume = {5},
	year = {2020},
	month = {04/2020},
	pages = {331-338},
	abstract = {In this paper, we present an optimization-based method for controlling aerial manipulators in physical contact with the environment. The multi-task control problem, which includes hybrid force-motion tasks, energetic tasks, and position/postural tasks, is recast as a quadratic programming problem with equality and inequality constraints, which is solved online. Thanks to this method, the aerial platform can be exploited at its best to perform the multi-objective tasks, with tunable priorities, while hard constraints such as contact maintenance, friction cones, joint limits, maximum and minimum propeller speeds are all respected. An on-board force/torque sensor mounted at the end effector is used in the feedback loop in order to cope with model inaccuracies and reject external disturbances. Real experiments with a multi-rotor platform and a multi-DoF lightweight manipulator demonstrate the applicability and effectiveness of the proposed approach in the real world.},
	keywords = {Aerial Physical Interaction, Aerial Robotics},
	doi = {10.1109/LRA.2019.2958473},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2020a-NavSabTogPucFra.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2020a-NavSabTogPucFra.mp4},
	author = {Gabriele Nava and Quentin Sabl{\'e} and Marco Tognon and Daniele Pucci and Antonio Franchi}
}
@article {2018t-MasMohRobFra,
	title = {Shared Planning and Control for Mobile Robots with Integral Haptic Feedback},
	journal = {The International Journal of Robotics Research},
	volume = {37},
	year = {2018},
	month = {10/2018},
	pages = {1395-1420},
	abstract = {This paper presents a novel bilateral shared framework for online trajectory generation for mobile robots. The robot navigates along a dynamic path, represented as a B-spline, whose parameters are jointly controlled by a human supervisor and by an autonomous algorithm. The human steers the reference (ideal) path by acting on the path parameters which are also affected, at the same time, by the autonomous algorithm in order to ensure: i) collision avoidance, ii) path regularity and iii) proximity to some points of interest. These goals are achieved by combining a gradient descent-like control action with an automatic algorithm that re-initializes the traveled path (replanning) in cluttered environments in order to mitigate the effects of local minima. The control actions of both the human and the autonomous algorithm are fused via a filter that preserves a set of local geometrical properties of the path in order to ease the tracking task of the mobile robot. The bilateral component of the interaction is implemented via a force feedback that accounts for both human and autonomous control actions along the whole path, thus providing information about the mismatch between the reference and traveled path in an integral sense. The proposed framework is validated by means of realistic simulations and actual experiments deploying a quadrotor UAV supervised by a human operator acting via a force-feedback haptic interface. Finally, a user study is presented in order to validate the effectiveness of the proposed framework and the usefulness of the provided force cues.},
	keywords = {Aerial Robotics, Optimal Trajectory Planning},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2018t-MasMohRobFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2018t-MasMohRobFra-video1.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2018t-MasMohRobFra-video2.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2018t-MasMohRobFra-video3.mp4},
	author = {Carlo Masone and Mostafa Mohammadi and Paolo Robuffo Giordano and Antonio Franchi}
}
@inbook {2013n-SpeNotBueFra,
	title = {Aggressive Maneuver Regulation of a Quadrotor UAV},
	booktitle = {Robotics Research, The 16th International Symposium ISRR},
	volume = {114},
	number = {Springer Tracts in Advanced Robotics},
	year = {2016},
	month = {04/2016},
	pages = {95-112},
	publisher = {Springer},
	organization = {Springer},
	abstract = {In this paper we design a nonlinear controller for aggressive maneuvering of a quadrotor. We take a maneuver regulation perspective. Differently from the classical trajectory tracking approach, maneuver regulation does not require following a timed reference state, but a geometric {\textquotedblleft}path{\textquotedblright} with a velocity (and possibly orientation) profile assigned on it. The proposed controller re- lies on three main ideas. Given a desired maneuver, i.e., a set of state trajectories equivalent under time translations, the system dynamics is decomposed into dynamics longitudinal and transverse to the maneuver. A space-dependent version of the transverse dynamics is derived, by using the longitudinal state, i.e., the arc-length of the path, as an independent variable. Then the controller is obtained as a function of the arc-length consisting of two terms: a feedforward term, being the nominal input to apply when on the path at the current arc-length, and a feedback term exponentially stabilizing the state-dependent transverse dynamics. Numerical computations are presented to prove the effectiveness of the proposed strategy. The controller performances are tested in presence of uncertainty of the model parameters and input noise and saturations. The controller is also tested in a realistic simulation environment validated against an experimental test-bed.},
	keywords = {Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-1_no_sat.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-1_no_sat_zoom.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-2_sat_8_5.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-2_sat_8_5_zoom.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-2_sat_7.mp4 , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013n-SpeNotBueFra-2_sat_7_zoom.mp4},
	author = {Sara Spedicato and Giuseppe Notarstefano and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2016a-SpeFraNot,
	title = {From Tracking to Robust Maneuver Regulation: an Easy-to-Design Approach for VTOL Aerial Robots},
	booktitle = {2016 IEEE Int. Conf. on Robotics and Automation},
	year = {2016},
	month = {05/2016},
	pages = {2965-2970},
	address = {Stockholm, Sweden},
	abstract = {In this paper we present a maneuver regulation scheme for Vertical Take-Off and Landing (VTOL) micro aerial vehicles (MAV). Differently from standard trajectory tracking, maneuver regulation has an intrinsic robustness due to the fact that the vehicle is not required to chase a virtual target, but just to stay on a (properly designed) desired path with a given velocity profile. In this paper we show how a robust maneuver regulation controller can be easily designed by converting an existing tracking scheme. The resulting maneuvering controller has three main appealing features, namely it: (i) inherits the robustness properties of the tracking controller, (ii) gains the appealing features of maneuver regulation, and (iii) does not need any additional tuning with respect to the tracking controller. We prove the correctness of the proposed scheme and show its effectiveness in experiments on a nano-quadrotor. In particular, we show on a nontrivial maneuver how external disturbances acting on the quadrotor cause instabilities in the standard tracking, while marginally affect the maneuver regulation scheme.},
	keywords = {Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016a-SpeFraNot-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016a-SpeFraNot.mp4},
	author = {Sara Spedicato and Antonio Franchi and Giuseppe Notarstefano}
}
@article {2016c-TogDasFra,
	title = {Observer-based Control of Position and Tension  for an Aerial Robot Tethered to a Moving Platform},
	journal = {IEEE Robotics and Automation Letters},
	volume = {1},
	year = {2016},
	note = {Also selected for presentation at the 2016 IEEE Int. Conf. on Robotics and Automation, Stockholm , Sweden},
	month = {01/2016},
	pages = {732-737},
	abstract = {In this paper we address a challenging version of the problem of controlling tethered aerial vehicles (also known as UAV, MAV, and UAS) by considering the aerial robot linked to a generic and independently moving platform. We solve the exact tracking control problem for both the 3D position of the robot (either absolute or with respect to the platform) and the tension along the link. To achieve this goal we prove some fundamental system properties, useful to design a nonlinear controller, such as differential flatness and dynamic feedback linearizability. To close the control loop a set of minimal and standard sensors is proposed. Then we show that it is possible to retrieve the full system state from those sensors by means of nonlinear measurements transformations and a bank of low-dimension estimators based on the nonlinear high gain observer. The ability of the proposed observer-controller method is tested by extensive numerical simulations spanning many non-ideal conditions.},
	keywords = {Aerial Physical Interaction, Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016c-TogDasFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2016c-TogDasFra..mp4},
	author = {Marco Tognon and Sanket S. Dash and Antonio Franchi}
}
@conference {2015f-YueMahSecBueFra,
	title = {Design, Identification and Experimental Testing of a Light-Weight Flexible-joint Arm for Aerial Physical Interaction},
	booktitle = {2015 IEEE Int. Conf. on Robotics and Automation},
	year = {2015},
	month = {05/2015},
	pages = {870-876},
	address = {Seattle, WA},
	abstract = {In this paper we introduce the design of a light- weight novel flexible-joint arm for light-weight unmanned aerial vehicles (UAVs), which can be used both for safe physical interaction with the environment and it represents also a preliminary step in the direction of performing quick motions for tasks such as hammering or throwing. The actuator consists of an active pulley driven by a rotational servo motor, a passive pulley which is attached to a rigid link, and the elastic connections (springs) between these two pulleys. We identify the physical parameters of the system, and use an optimal control strategy to maximize its velocity by taking advantage of elastic components. The prototype can be extended to a light-weight variable stiffness actuator. The flexible-joint arm is applied on a quadrotor, to be used in aerial physical interaction tasks, which implies that the elastic components can also be used for stable interaction absorbing the interactive disturbances which might damage the flying system and its hardware. The design is validated through several experiments, and future developments are discussed in the paper.},
	keywords = {Aerial Physical Interaction, Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015f-YueMahSecBueFra-preprint_0.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015f-YueMahSecBueFra.mp4},
	author = {Burak Y{\"u}ksel and Saber Mahboubi and Cristian Secchi and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2015c-GioMohFraPra,
	title = {A Force-based Bilateral Teleoperation Framework for Aerial Robots in Contact with the Environment},
	booktitle = {2015 IEEE Int. Conf. on Robotics and Automation},
	year = {2015},
	month = {05/2015},
	pages = {318-324},
	address = {Seattle, WA},
	keywords = {Aerial Physical Interaction, Aerial Robotics, Bilateral Shared Control of Mobile Robots},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015c-GioMohFraPra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015c-GioMohFraPra.mp4},
	author = {Guido Gioioso and Mostafa Mohammadi and Antonio Franchi and Domenico Prattichizzo}
}
@conference {2015e-RajRylBueFra,
	title = {Modeling, Control and Design Optimization for a Fully-actuated Hexarotor Aerial Vehicle with Tilted Propellers},
	booktitle = {2015 IEEE Int. Conf. on Robotics and Automation},
	year = {2015},
	month = {05/2015},
	pages = {4006-4013},
	address = {Seattle, WA},
	keywords = {Aerial Robotics, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015e-RajRylBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015e-RajRylBueFra.mp4},
	author = {Sujit Rajappa and Markus Ryll and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2015j-TogFra,
	title = {Nonlinear Observer for the Control of Bi-Tethered Multi Aerial Robots},
	booktitle = {2015 IEEE/RSJ Int. Conf. on Intelligent Robots and Systems},
	year = {2015},
	month = {09/2015},
	pages = {1852-1857},
	address = {Hamburg, Germany},
	abstract = {We consider the problem of state-observation and control for a bi-tethered aerial system composed by a physical chain of two underactuated aerial robots, also called UAVs.  The controlled outputs are the Cartesian position of the last robot and the internal forces along the links. We aim at a minimal use of sensors in order to retrieve the full state. For this goal we propose an output transformation method whose applicability implies the system observability.  When this is the case we prove that it is possible to design a nonlinear state estimator based on the  high gain- and Luenberger-  observers that is able to retrieve the state from any dynamic condition. We also demonstrate how this estimator can be employed with a nonlinear controller for the Cartesian position and the link stresses while ensuring the stability in  closed-loop. We show the validity of the method for sensorial configurations composed only by two accelerometers (no gyros) and just two encoders, or two accelerometers (no gyros) and just two inclinometers. A realistic simulative validation concludes the paper.},
	keywords = {Aerial Physical Interaction, Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015j-TogFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2015j-TogFra.mp4},
	author = {Marco Tognon and Antonio Franchi}
}
@conference {2014a-GioFraSalSchPra,
	title = {The Flying Hand: a Formation of UAVs for Cooperative Aerial Tele-Manipulation},
	booktitle = {2014 IEEE Int. Conf. on Robotics and Automation},
	year = {2014},
	month = {05/2014},
	pages = {4335-4341},
	address = {Hong Kong, China},
	abstract = {The flying hand is a robotic hand consisting of a swarm of UAVs able to grasp an object where each UAV contributes to the grasping task with a single contact point at the tooltip. The swarm of robots is teleoperated by a human hand whose fingertip motions are tracked, e.g., using an RGB-D camera. We solve the kinematic dissimilarity of this unique master-slave system using a multi-layered approach that includes: a hand interpreter that translates the fingertip motion in a desired motion for the object to be manipulated; a mapping algorithm that transforms the desired object motions into a suitable set of virtual points deviating from the planned contact points; a compliant force control for the case of quadrotor UAVs that allows to use them as indirect 3D force effectors. Visual feedback is also used as sensory substitution technique to provide a hint on the internal forces exerted on the object. We validate the approach with several human-in-the-loop simulations including the full physical model of the object, contact points and UAVs.},
	keywords = {Aerial Physical Interaction, Aerial Robotics, Bilateral Shared Control of Mobile Robots, Motion control of multiple robots},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014a-GioFraSalSchPra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014a-GioFraSalSchPra.mp4},
	author = {Guido Gioioso and Antonio Franchi and Gionata Salvietti and Stefano Scheggi and Domenico Prattichizzo}
}
@conference {2014g-GagOriBueFra,
	title = {Image-based Road Network Clearing without Localization and without Maps using a Team of UAVs},
	booktitle = {2014 European Control Conference},
	year = {2014},
	month = {06/2014},
	pages = {1902-1908},
	address = {Strasbourg, France},
	abstract = {We address the problem of clearing an arbitrary and unknown network of roads using an organized team of Unmanned Aerial Vehicles (UAVs) equipped with a monocular down-facing camera, an altimeter, plus high-bandwidth short- range and low-bandwidth long-range communication systems. We allow the UAVs to possibly split in several subgroups. In each subgroup a leader guides the motion employing a hierarchical coordination. A feature/image-based algorithm guides the subgroup toward the unexplored region without any use of global localization or environmental mapping. At the same time all the entry-points of the the explored region are kept under control, so that any moving object that enters or exits the previously cleared area. Simulative results on real aerial images demonstrate the functionalities and the effectiveness of the proposed algorithm. },
	keywords = {Aerial Robotics, Exploration, Motion control of multiple robots, Patrolling / Surveillance, Pursuit-evasion / Clearing},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014g-GagOriBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014g-GagOriBueFra-video.mp4},
	author = {Matteo Gagliardi and Giuseppe Oriolo and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2014d-YueSecBueFra,
	title = {Reshaping the Physical Properties of a Quadrotor through IDA-PBC and its Application to Aerial Physical Interaction},
	booktitle = {2014 IEEE Int. Conf. on Robotics and Automation},
	year = {2014},
	month = {05/2014},
	pages = {6258-6265},
	address = {Hong Kong, China},
	abstract = {In this paper we propose a controller, based on an extension of Interconnection and Damping Assignment Passivity Based Control (IDA-PBC) framework, for shaping the whole physical characteristics of a quadrotor and for obtaining a desired interactive behavior between the robot and the environment. In the control design, we shape the total energy (kinetic and potential) of the undamped original system by first excluding external effects. In this way we can assign a new dynamics to the system. Then we apply damping injection to the new system for achieving a desired damped behavior. Then we show how to connect a high-level control input to such system by taking advantage of the new desired physics. We support the theory with extensive simulations by changing the overall behavior of the UAV for different desired dynamics, and show the advantage of this method for sliding on a surface tasks, such as ceiling painting, cleaning or surface inspection.},
	keywords = {Aerial Physical Interaction, Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014d-YueSecBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014d-YueSecBueFra.mp4},
	author = {Burak Y{\"u}ksel and Cristian Secchi and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2014e-SteBasBueFra,
	title = {A Semi-autonomous UAV Platform for Indoor Remote Operation with Visual and Haptic Feedback},
	booktitle = {2014 IEEE Int. Conf. on Robotics and Automation},
	year = {2014},
	month = {05/2014},
	pages = {3862-3869},
	address = {Hong Kong, China},
	abstract = {We present the development of a semi-autonomous quadrotor UAV platform for indoor teleoperation using RGB- D technology as exteroceptive sensor. The platform integrates IMU and Dense Visual Odometry pose estimation in order to stabilize the UAV velocity and track the desired velocity commanded by a remote operator though an haptic inter- face. While being commanded, the quadrotor autonomously performs a persistent pan-scanning of the surrounding area in order to extend the intrinsically limited field of view. The RGB-D sensor is used also for collision-safe navigation using a probabilistically updated local obstacle map. In the operator visual feedback, pan-scanning movement is real time compensated by an IMU-based adaptive filtering algorithm that lets the operator perform the drive experience in a oscillation- free frame. An additional sensory channel for the operator is provided by the haptic feedback, which is based on the obstacle map and velocity tracking error in order to convey information about the environment and quadrotor state. The effectiveness of the platform is validated by means of experiments performed without the aid of any external positioning system.},
	keywords = {Aerial Robotics, Localization of aerial robots, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014e-SteBasBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014e-SteBasBueFra.mp4},
	author = {Paolo Stegagno and Massimo Basile and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2014b-GioRylPraBueFra,
	title = {Turning a Near-hovering Controlled Quadrotor into a 3D Force Effector},
	booktitle = {2014 IEEE Int. Conf. on Robotics and Automation},
	year = {2014},
	month = {05/2014},
	pages = {6278-6284},
	address = {Hong Kong, China},
	abstract = {In this paper the problem of a quadrotor that physically interacts with the surrounding environment through a rigid tool is considered. We present a theoretical design that allows to exert an arbitrary 3D force by using a standard near-hovering controller that was originally developed for contact-free flight control. This is achieved by analytically solving the nonlinear system that relates the quadrotor state, the force exerted by the rigid tool on the environment, and the near-hovering controller action at the equilibrium points, during any generic contact. Stability of the equilibria for the most relevant actions (pushing, releasing, lifting, dropping, and left-right shifting) are proven by means of numerical analysis using the indirect Lyapunov method. An experimental platform, including a suitable tool design, has been developed and used to validate the theory with preliminary experiments.},
	keywords = {Aerial Physical Interaction, Aerial Robotics, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014b-GioRylPraBueFra-preprint.pdf , https://homepages.laas.fr/afranchi/robotics/sites/default/files/2014b-GioRylPraBueFra.mp4},
	author = {Guido Gioioso and Markus Ryll and Domenico Prattichizzo and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2013o-SpiRobRylBueFra,
	title = {An Open-Source Hardware/Software Architecture for Quadrotor UAVs},
	booktitle = {2nd IFAC Workshop on Research, Education and Development of Unmanned Aerial Systems},
	year = {2013},
	month = {11/2013},
	address = {Compiegne, France},
	abstract = {In this paper, we illustrate an open-source ready-to-use hardware/software architecture for a quadrotor UAV. The presented platform is price effective, highly customizable, and easily exploitable by other researchers involved in high-level UAV control tasks and for educational purposes as well. The use of object-oriented programming and full support of Robot Operating System (ROS) and Matlab Simulink allows for an efficient customization, code reuse, functionality expansion and rapid prototyping of new algorithms. We provide an extensive illustration of the various UAV components and a thorough description of the main basic algorithms and calibration procedures. Finally, we present some experimental case studies aimed at showing the effectiveness of the proposed architecture.},
	keywords = {Aerial Robotics, UAV hardware platforms},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013o-SpiRobRylBueFra-preprint.pdf},
	author = {Riccardo Spica and Paolo Robuffo Giordano and Markus Ryll and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2013q-SteBasBueFra,
	title = {RGB-D based Haptic Teleoperation of UAVs with Onboard Sensors: Development and Preliminary Results},
	booktitle = {2013 IROS Work. on Vision-based Closed-Loop Control and Navigation of Micro Helicopters in GPS-denied Environments},
	year = {2013},
	month = {11/2013},
	address = {Tokyo, Japan},
	keywords = {Aerial Robotics, UAV hardware platforms},
	author = {Paolo Stegagno and Massimo Basile and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
@conference {2013r-Fra,
	title = {Towards Autonomous-collaborative and Haptic-tele-operated UAVs with Fully-onboard State Estimation and Physical Interaction Capabilities},
	booktitle = {2013 IROS Work. on From Remotely-Controlled to Autonomous-Collaborative Robots},
	year = {2013},
	month = {11/2013},
	address = {Tokyo, Japan},
	keywords = {Aerial Physical Interaction, Aerial Robotics, UAV hardware platforms},
	author = {Antonio Franchi}
}
@conference {2013p-SteBasBueFra,
	title = {Vision-based Autonomous Control of a Quadrotor UAV using an Onboard RGB-D Camera and its Application to Haptic Teleoperation},
	booktitle = {2nd IFAC Workshop on Research, Education and Development of Unmanned Aerial Systems},
	year = {2013},
	month = {11/2013},
	address = {Compiegne, France},
	keywords = {Aerial Robotics},
	attachments = {https://homepages.laas.fr/afranchi/robotics/sites/default/files/2013p-SteBasBueFra-preprint.pdf},
	author = {Paolo Stegagno and Massimo Basile and Heinrich H. B{\"u}lthoff and Antonio Franchi}
}
