Cutting a unit square and permuting blocks

Abstract: Consider a random permutation of $kn$ objects that permutes $n$ disjoint blocks of size $k$ and then permutes elements within each block. Normalizing its cycle lengths by $kn$ gives a random partition of unity, and we derive the limit law of this partition as $k,n \to \infty$. The limit may be constructed via a simple square cutting procedure that generalizes stick breaking in the classical case of uniform permutations ($k=1$). The proof is by coupling, providing upper and lower bounds on the Wasserstein distance. The limit is shown to have a certain self-similar structure that gives the distribution of large cycles and relates to a multiplicative function involving the Dickman function. Along the way we also give the first extension of the Erd\H{o}s-Tur\'an law to a proper permutation subgroup.