https://www.boeckler.de/pdf/p_imk_report_152e_2020.pdf
https://www.raadvanstate.nl/publicaties/publicaties.html
https://www.propublica.org/article/how-private-nonprofits-ultrawealthy-tax-deductions-museums-foundation-art
https://www.ippr.org/files/2018-03/cej-our-common-wealth-march2018.pdf