alt Hacker NewsThere is idea behind that, but continuous is not enough.
The variable is all transfers, taxes and benefits T = [all taxes - all benefits] as function of income per person (including children). T starts negative (benefits are negative taxes).
Goal: monotonously increasing effective marginal T rate.
This. No need for continuity in the derivative - the marginal tax rate should be fine to jump around - but should start low (I'm fine with negative) and increase as you earn more. Like US federal tax brackets, but with benefits also considered.