http://www.iemed.org/observatori/arees-danalisi/arxius-adjunts/anuari/med.2018/IEMed_Crisis_EU_project_Medyearbook2018_Marc_Pierini.pdf
http://www.aviationlaw.eu/wp/wp-content/uploads/2013/09/Freedoms-of-the-Air-Explained.pdf
https://www.lobbycontrol.de/wp-content/uploads/gaslobby-studie-lobbycontrol.pdf