https://doi.org/10.1351/goldbook.S05502

In an assembly of macromolecules, a continuous distribution with the differential mass-distribution function of the form: \[f_{\text{w}}(x)\ \text{d}x = \frac{a^{b\,+\,1}}{\varGamma\left(b\,+\,1\right)}\ x^{b}\ \text{e}^{-a\ x}\ \text{d}x\] 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, 1^{st} ed., p. 56 (http://old.iupac.org/publications/books/author/metanomski.html)

Purple Book, 1