https://www.diw.de/documents/publikationen/73/diw_01.c.670904.de/19-31-3.pdf
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwij8PLy0aH1AhVEhP0HHf7rAYwQFnoECAYQAQ&url=https%3A%2F%2Fassets.gov.ie%2F203426%2F468690ef-2b01-4d7d-a6d3-c0cbabcab3cb.pdf&usg=AOvVaw2-vnoHbS5dgbnNXNla3sLK
http://ec.europa.eu/transport/sites/transport/files/eu_air_carriers_by_country_holding_an_active_operating_licence.pdf
https://data.consilium.europa.eu/doc/document/ST-14065-2022-INIT/en/pdf