https://taxfoundation.org/flat-tax-state-income-tax-reform/
https://www.brookings.edu/blog/future-development/2022/06/27/targeted-legislative-tweaks-can-help-contain-the-harm-of-debt-crises/?utm_campaign=Brookings%20Brief&utm_medium=email&utm_content=218267785&utm_source=hs_email
http://www.case-research.eu/files/?id_plik=5223
https://www.bertelsmann-stiftung.de/fileadmin/files/BSt/Publikationen/GrauePublikationen/20200325_PEPP_GrundII.pdf