http://www.diw.de/documents/publikationen/73/diw_01.c.562050.de/17-29.pdf
https://theconversation.com/eu-enlargement-what-does-the-future-hold-227716
UKIN-State-of-the-EU-Report-1.pdf (ukandeu.ac.uk)
https://www.consilium.europa.eu/media/ny3j24sm/much-more-than-a-market-report-by-enrico-letta.pdf