Von Bertalanffy function

The von Bertalanffy growth function (VBGF), or von Bertalanffy curve, is a type of growth curve for a time series and is named after Ludwig von Bertalanffy. It is a special case of the generalised logistic function. The growth curve (biology) is used to model mean length from age in animals. The function is commonly applied in ecology to model fish growth.
The model can be written as the following:
where formula_2 is age, formula_3 is the growth coefficient, formula_4 is a value used to calculate size when age is zero, and formula_5 is asymptotic size. It is the solution of the following linear differential equation:
Seasonally-adjusted von Bertalanffy.
The seasonally-adjusted von Bertalanffy is an extension of this function that accounts for organism growth that occurs seasonally. It was created by I. F. Somer in 1988.