Utility used by life.Leslie to graduate the person-years lived by those dying in the interval by the
method of Keyfitz and Flieger (1990).
keyfitz(b0, nMx)Two-element vector of regression coefficients provided in Keyfitz and Flieger (1990).
Period central death rates = nDx/nKx.
The first two values (age classes 0-1 and 1-5) of the nax column of a period life table.
Keyfitz, N. and Flieger, W. (1990). World population growth and aging: Demographic trends in the late twentieth century. Chicago: University of Chicago Press.