Decomposition of free cumulants

Abstract: Free cumulants are multilinear functionals defined in terms of the moment functional with the use of the family of lattices of noncrossing partitions. In the univariate case, they can be identified with the coefficients of the Voiculescu transform of the moment functional which plays a role similar to that of the logarithm of the Fourier transform. The associated linearization property is connected with free independence. In turn, the family of much smaller lattices of interval partitions is used to define Boolean cumulants connected with Boolean independence. In order to bridge the gap between these two families of lattices and the associated cumulants we introduce and study the family of lattices of noncrossing partitions adapted to Motzkin paths and define the associated operator-valued `Motzkin cumulants'. We prove the corresponding M\"{o}bius inversion formula which plays the role of a lattice refinement of the formula expressing free cumulants in terms of Boolean cumulants. We apply this concept to free probability and obtain the additive decomposition of free cumulants in terms of scalar-valued counterparts of Motzkin cumulants.