This lifetable identity is the same no matter what kind of lifetable is required. You can find it in any demography textbook.
numeric. Vector of age-specific lifetable survivorship.
ndx vector of lifetable deaths.
The vector returned is the same length as
lx and it sums to the lifetable radix.
If the radix is one then this is the discrete deaths distribution.
Preston S, Heuveline P, Guillot M (2000). Demography: measuring and modeling population processes. Wiley-Blackwell.