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The differences between the number of survivors (432-4) at different ages gives the number of deaths within the given age interval of the death function ¹. Life tables typically include the expectation of life ³ or life expectancy ³ at age x ; this is the mean number of years to be lived by those surviving to exact age x, given the mortality conditions of the table. The expectation of life at birth ⁴ is a particular case of expectation of life, and represents the mean length of life ⁴ of individuals who have been subjected since birth to the mortality of the table. The reciprocal of the expectation of life at birth is the life table death rate ⁵ or death rate of the stationary population ⁵.

• 3. By integrating the survivorship function (432-3) between two exact given ages we obtain the total number of years lived by the cohort between these ages; the notation for the total number of years lived between age x and x + n is _nL_x . This function is often called the stationary population in life table column headings. By summing it from a given age x to the end of life, we obtain the total number of years to be lived after attaining age x by those reaching that age; the conventional notation is T _x .
• 4. The notation for the expectation of life at age x is e_x

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