https://www.socialeurope.eu/2017/07/49234/
https://theconversation.com/quien-puede-convertirse-en-ciudadano-espanol-vericuetos-legales-para-obtener-la-nacionalidad-223771
http://cadmus.eui.eu/bitstream/handle/1814/45187/Ebook_IntegrationMigrantsRefugees2017.pdf?sequence=3&isAllowed=y
http://www.ecreu.com/pdfs/alternative-white-paper-presented-by-UK-%20citizens-in-europe.pdf