https://www.nber.org/system/files/working_papers/w28598/w28598.pdf
https://taxfoundation.org/flat-tax-state-income-tax-reform/
http://www.oecd-ilibrary.org/docserver/download/5jlv2jhfnt48-en.pdf?expires=1494939516&id=id&accname=guest&checksum=08592067325A17BFB82AC6F6E15110B2
https://www.imf.org/-/media/Files/Publications/PP/2023/English/PPEA2023001.ashx