Troupes, Cumulants, and Stack-Sorting

Several sequences of free cumulants that count binary plane trees correspond to sequences of classical cumulants that count the decreasing versions of the same trees. Using two new operations on colored binary plane trees that we call insertion and decomposition, we prove that this surprising phenomenon holds for families of trees that we call troupes. We give a simple characterization of troupes, which provide a broad framework for generalizing several of the results known about West's stack-sorting map $s$. Indeed, we give new proofs of some of the main techniques that have been developed for understanding $s$; these new proofs are far more conceptual than the original ones, explain how the objects called valid hook configurations arise naturally, and generalize to troupes. For $t\in\{2,3\}$, we enumerate $t$-stack-sortable alternating permutations of odd length and $t$-stack-sortable permutations whose descents are all peaks. The unexpected connection between troupes and cumulants provides a powerful new tool for analyzing the stack-sorting map that hinges on free probability theory. We give numerous applications of this method. For example, we show that if $\sigma\in S_{n-1}$ is chosen uniformly at random, then the expected value of $\text{des}(s(\sigma))+1$ is $$\left(3-\sum_{j=0}^n\frac{1}{j!}\right)n.$$ Furthermore, the variance of $\text{des}(s(\sigma))+1$ is asymptotically $(2+2e-e^2)n$. We obtain similar results concerning the expected number of descents of postorder readings of decreasing colored binary plane trees. We also obtain improved estimates for $|s(S_n)|$ and an improved lower bound for the degree of noninvertibility of $s$. We give two novel formulas that convert from free to classical cumulants. The first is given by a sum over noncrossing partitions, and the second is given by a sum over $231$-avoiding valid hook configurations. We pose several open problems.