Operads of (noncrossing) partitions, interacting bialgebras, and moment-cumulant relations
arXiv:1907.01190 · doi:10.1016/j.aim.2020.107170
Abstract
We establish and explore a relationship between two approaches to moment-cumulant relations in free probability theory: on one side the main approach, due to Speicher, given in terms of Möbius inversion on the lattice of noncrossing partitions, and on the other side the more recent non-commutative shuffle-algebra approach, where the moment-cumulant relations take the form of certain exponential-logarithm relations. We achieve this by exhibiting two operad structures on (noncrossing) partitions, different in nature: one is an ordinary, non-symmetric operad whose composition law is given by insertion into gaps between elements, the other is a coloured, symmetric operad with composition law expressing refinement of blocks. We show that these operad structures interact so as to make the corresponding incidence bialgebra of the former a comodule bialgebra for the latter. Furthermore, this interaction is compatible with the shuffle structure and thus unveils how the two approaches are intertwined. Moreover, the constructions and results are general enough to extend to ordinary set partitions.
arXiv comment41 pages. v4: Fixed a small mistake on p.23; it does not affect the rest of the paper. Final version, to appear in Adv. Math
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- Shuffle Algebras and Non-Commutative Probability for Pairs of Faces