
https://www.diw.de/documents/publikationen/73/diw_01.c.578096.de/18-7-3.pdf

https://www.opendemocracy.net/en/dark-money-investigations/laws-protecting-britains-democracy-from-big-money-are-broken/

http://www.oecd-ilibrary.org/docserver/download/5jlv2jhfnt48-en.pdf?expires=1494939516&id=id&accname=guest&checksum=08592067325A17BFB82AC6F6E15110B2