“The Distribution of N-Grams”, Leo Egghe2000-02 (language, probability theory)⁠:

n-grams are generalized words consisting of n consecutive symbols, as they are used in a text. This paper determines the rank-frequency distribution for redundant n-grams. For entire texts this is known to be Zipf’s law (ie. an inverse power law). For n-grams, however, we show that the rank (r)-frequency distribution is

P_n(r) = C / (Ψ_N (r))^β

where ψ_N is the inverse function of f_N(10) = x ln^N−1x. (Here we assume that the rank-frequency distribution of the symbols follows Zipf’s law with exponent β.)

[Sums of power laws are not power laws, so can be quite different.]