The TDI is the (1) absolute value of the n-day sum of the n-day
momentum, minus the quantity of (2) multiple*n-day sum of the
absolute value of the n-day momentum, minus (3) n-day sum of
the absolute value of the n-day momentum.
I.e. TDI = (1) - [ (2) - (3) ]
The direction indicator is the sum of the n-day momentum over the last
n days.
See URL in references section for further details.