Schulz–Zimm distribution

In an assembly of macromolecules, a continuous distribution with the differential mass-distribution function of the form:

$f w x d x = a b + 1 Γ b + 1 x b e − a x d x$

where $x$ is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization, $a$ and $b$ are positive adjustable parameters, and ( $Γ b + 1$ ) is the gamma function of ( $b + 1$ ).

Source:

Purple Book, p. 56