From higher order free cumulants to non-separable hypermaps

Abstract: Higher order free moments and cumulants, introduced by Collins, Mingo, \'Sniady and Speicher in 2006, describe the fluctuations of unitarily invariant random matrices in the limit of infinite size. The functional relations between their generating functions were only found last year by Borot, Garcia-Failde, Charbonnier, Leid and Shadrin and a combinatorial derivation is still missing. We simplify these relations and show how their combinatorial derivation reduces to the computation of generating functions of planar non-separable hypermaps with prescribed vertex valencies and weighted hyper-edges. The functional relations obtained by Borot et al. involve some remarkable simplifications, which can be formulated as identities satisfied by these generating functions. The case of third order free cumulants, whose combinatorial understanding was already out of reach, is derived explicitly.