A differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders...
, where N is the quantity and λ (lambda) is a positive number called the decay constant:
The solution to this equation (see derivation below) is:
Exponential rate of change
Here N(t) is the quantity at time t, and N0 = N(0) is the initial quantity, i.e.