most probable distribution (in macromolecular assemblies)

https://doi.org/10.1351/goldbook.M04035
A discrete distribution with the differential mass-distribution function of the form: \[f_{\text{w}}(x) = a^{2}\ x\ (1- a)^{x- 1}\] where x is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization and a is a positive adjustable parameter. For large values of x, the most probable distribution converges to the particular case of the Schulz–Zimm distribution with b = 1. In the literature, this distribution is sometimes referred to as the Flory distribution or the Schulz–Flory distribution.
Source:
Purple Book, 1st ed., p. 56 (http://old.iupac.org/publications/books/author/metanomski.html)