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Equation 3 · Part 2 · How Demography, Migration, and Urban Futures Actually Work

Symbol P_a,t

Pa+1,t+1=Pa,t Sa,t+Ma,tP_{a+1,t+1} = P_{a,t}\, S_{a,t} + M_{a,t}
Pa,tP_{a,t}

What this part means

the population in age group a at time t.

Its job in the formula

PaP_a,t is one of the signed contributions combined to compute the quantity on the left.

Where the article explains it

where Pa,tP_{a,t} is the population in age group a at time t , Sa,tS_{a,t} is the survival rate applied to that cohort, and Ma,tM_{a,t} is net migration entering or leaving that age group during the interval.

The passage around this formula

…rates applied to women of reproductive age (which generates the next year’s births), and net international migration assumptions applied by age [ 1 ] . Formally, for an age group a in year t : Pa+1,t+1=Pa,t Sa,t+Ma,tP_{a+1,t+1} = P_{a,t}\, S_{a,t} + M_{a,t}. where Pa,tP_{a,t} is the population in age group a at time t , Sa,tS_{a,t} is the survival rate applied to that cohort, and Ma,tM_{a,t} is net migration entering or leaving that age group during the interval. Births are generated separately by summing age-specific fertility rates across women of childbearing age,…

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Learn the underlying idea

A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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Sources cited in the surrounding passage

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