Returns the population number in age interval [Age,Age+AgeInterval). If AgeInterval is not specified it defaults to 1.0. In terms of the instantaneous population density \(p(x)\) the number \(_n p_x\) from Age \(x\) for AgeInterval \(n\) is $$ _n p_x = \int_x^{x+n} p(a) da $$
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