Citation:
d^2B/dD^2 represents the second derivative of B(t) with respect to D(t). In this context, it measures how the rate at which births occur changes as deaths occur.
Also known as the derivative, it measures how a function changes with respect to its independent variable.
The process of finding derivatives of functions.
Rate of Change of Rates: A measure of how the rate at which something is changing itself changes over time.