21 research outputs found
Star-cumulants of free unitary Brownian motion
We study joint free cumulants of u_t and u_t^{*}, where u_t is a free unitary
Brownian motion at time t. We determine explicitly some special families of
such cumulants. On the other hand, for a general joint cumulant of u_t and
u_t^{*}, we "calculate the derivative" for t going to infinity, when u_t
approaches a Haar unitary. In connection to the latter calculation we put into
evidence an "infinitesimal determining sequence" which naturally accompanies an
arbitrary R-diagonal element in a tracial *-probability space.Comment: 35 pages. This version has added details in Sections 5 and
Structure and enumeration of (3+1)-free posets
A poset is (3+1)-free if it does not contain the disjoint union of chains of
length 3 and 1 as an induced subposet. These posets play a central role in the
(3+1)-free conjecture of Stanley and Stembridge. Lewis and Zhang have
enumerated (3+1)-free posets in the graded case by decomposing them into
bipartite graphs, but until now the general enumeration problem has remained
open. We give a finer decomposition into bipartite graphs which applies to all
(3+1)-free posets and obtain generating functions which count (3+1)-free posets
with labelled or unlabelled vertices. Using this decomposition, we obtain a
decomposition of the automorphism group and asymptotics for the number of
(3+1)-free posets.Comment: 28 pages, 5 figures. New version includes substantial changes to
clarify the construction of skeleta and the enumeration. An extended abstract
of this paper appears as arXiv:1212.535
Toda Equations and Piecewise Polynomiality for Mixed Double Hurwitz Numbers
This article introduces mixed double Hurwitz numbers, which interpolate
combinatorially between the classical double Hurwitz numbers studied by
Okounkov and the monotone double Hurwitz numbers introduced recently by
Goulden, Guay-Paquet and Novak. Generalizing a result of Okounkov, we prove
that a certain generating series for the mixed double Hurwitz numbers solves
the 2-Toda hierarchy of partial differential equations. We also prove that the
mixed double Hurwitz numbers are piecewise polynomial, thereby generalizing a
result of Goulden, Jackson and Vakil
Polynomiality of monotone Hurwitz numbers in higher genera
Hurwitz numbers count branched covers of the Riemann sphere with specified
ramification, or equivalently, transitive permutation factorizations in the
symmetric group with specified cycle types. Monotone Hurwitz numbers count a
restricted subset of these branched covers, related to the expansion of
complete symmetric functions in the Jucys-Murphy elements, and have arisen in
recent work on the the asymptotic expansion of the
Harish-Chandra-Itzykson-Zuber integral. In previous work we gave an explicit
formula for monotone Hurwitz numbers in genus zero. In this paper we consider
monotone Hurwitz numbers in higher genera, and prove a number of results that
are reminiscent of those for classical Hurwitz numbers. These include an
explicit formula for monotone Hurwitz numbers in genus one, and an explicit
form for the generating function in arbitrary positive genus. From the form of
the generating function we are able to prove that monotone Hurwitz numbers
exhibit a polynomiality that is reminiscent of that for the classical Hurwitz
numbers, i.e., up to a specified combinatorial factor, the monotone Hurwitz
number in genus g with ramification specified by a given partition is a
polynomial indexed by g in the parts of the partition.Comment: 23 page