https://doi.org/10.1351/goldbook.C00982

The ratio of the mean-square end-to-end distance, < r2 >0, of a linear polymer chain in a theta state to N Ā· L 2, where N is the number of rigid sections in the main chain, each of length L; if all of the rigid sections are not of equal length, the mean-square value of L is used, i.e. \[L^{2}=\sum _{i}\frac{L_{i}^{\bar{2}}}{N}\] In simple single-strand chains, the bonds are taken as the rigid sections. The recommended symbol is: CN (Cā when N ā ā).*Source: *

Purple Book, 1^{st} ed., p. 49 (http://old.iupac.org/publications/books/author/metanomski.html)

Purple Book, 1