http://www.niesr.ac.uk/sites/default/files/publications/NIESR%20EU2%20MIGRATION%20REPORT.pdf
https://www.eurofound.europa.eu/sites/default/files/wpef17037.pdf
https://www.cer.eu/sites/default/files/insight_JS_JP_17.1.23.pdf
https://theconversation.com/quien-puede-convertirse-en-ciudadano-espanol-vericuetos-legales-para-obtener-la-nacionalidad-223771