So there is hope for the unemployed.
Discrete form
Above can also be formulated in discrete form. Indexation generally takes place with a lag, and then the discrete DMR is more adequate. This is:
DMR[y] = (T[y, q] - T[y-1 , q-1 ]) / (y - y-1 ) =
T /
y
Book III gives a development for the Bentham tax function, and also gives plots for regular numerical values. It appears that indexation and expectations about the growth of national income (relevant for indexation) again lead to other results than the conventional view on marginal rates.