28,926 research outputs found
Rational Combinatorics
We propose a categorical setting for the study of the combinatorics of
rational numbers. We find combinatorial interpretation for the Bernoulli and
Euler numbers and polynomials.Comment: Adv. in Appl. Math. (2007), doi:10.1016/j.aam.2006.12.00
Alexander Duality and Rational Associahedra
A recent pair of papers of Armstrong, Loehr, and Warrington and Armstrong,
Williams, and the author initiated the systematic study of {\em rational
Catalan combinatorics} which is a generalization of Fuss-Catalan combinatorics
(which is in turn a generalization of classical Catalan combinatorics). The
latter paper gave two possible models for a rational analog of the
associahedron which attach simplicial complexes to any pair of coprime positive
integers a < b. These complexes coincide up to the Fuss-Catalan level of
generality, but in general one may be a strict subcomplex of the other.
Verifying a conjecture of Armstrong, Williams, and the author, we prove that
these complexes agree up to homotopy and, in fact, that one complex collapses
onto the other. This reconciles the two competing models for rational
associahedra. As a corollary, we get that the involution (a < b)
\longleftrightarrow (b-a < b) on pairs of coprime positive integers manifests
itself topologically as Alexander duality of rational associahedra. This
collapsing and Alexander duality are new features of rational Catalan
combinatorics which are invisible at the Fuss-Catalan level of generality.Comment: 23 page
Unimodality Problems in Ehrhart Theory
Ehrhart theory is the study of sequences recording the number of integer
points in non-negative integral dilates of rational polytopes. For a given
lattice polytope, this sequence is encoded in a finite vector called the
Ehrhart -vector. Ehrhart -vectors have connections to many areas of
mathematics, including commutative algebra and enumerative combinatorics. In
this survey we discuss what is known about unimodality for Ehrhart
-vectors and highlight open questions and problems.Comment: Published in Recent Trends in Combinatorics, Beveridge, A., et al.
(eds), Springer, 2016, pp 687-711, doi 10.1007/978-3-319-24298-9_27. This
version updated October 2017 to correct an error in the original versio
Combinatorics of rational functions and Poincare-Birkhoff-Witt expansions of the canonical U(n-)-valued differential form
We study the canonical U(n-)-valued differential form, whose projections to
different Kac-Moody algebras are key ingredients of the hypergeometric integral
solutions of KZ-type differential equations and Bethe ansatz constructions. We
explicitly determine the coefficients of the projections in the simple Lie
albegras A_r, B_r, C_r, D_r in a conviniently chosen Poincare-Birkhoff-Witt
basis. As a byproduct we obtain results on the combinatorics of rational
functions, namely non-trivial identities are proved between certain rational
functions with partial symmetries.Comment: More typos correcte
Rational Dyck Paths in the Non Relatively Prime Case
We study the relationship between rational slope Dyck paths and invariant
subsets of extending the work of the first two authors in the
relatively prime case. We also find a bijection between --Dyck paths
and -tuples of -Dyck paths endowed with certain gluing data. These
are the first steps towards understanding the relationship between rational
slope Catalan combinatorics and the geometry of affine Springer fibers and knot
invariants in the non relatively prime case.Comment: 25 pages, 9 figure
Application of graph combinatorics to rational identities of type A
To a word , we associate the rational function . The main object, introduced by C. Greene to generalize
identities linked to Murnaghan-Nakayama rule, is a sum of its images by certain
permutations of the variables. The sets of permutations that we consider are
the linear extensions of oriented graphs. We explain how to compute this
rational function, using the combinatorics of the graph . We also establish
a link between an algebraic property of the rational function (the
factorization of the numerator) and a combinatorial property of the graph (the
existence of a disconnecting chain).Comment: This is the complete version of the submitted fpsac paper (2009
Ising n-fold integrals as diagonals of rational functions and integrality of series expansions
We show that the n-fold integrals of the magnetic susceptibility
of the Ising model, as well as various other n-fold integrals of the "Ising
class", or n-fold integrals from enumerative combinatorics, like lattice Green
functions, correspond to a distinguished class of function generalising
algebraic functions: they are actually diagonals of rational functions. As a
consequence, the power series expansions of the, analytic at x=0, solutions of
these linear differential equations "Derived From Geometry" are globally
bounded, which means that, after just one rescaling of the expansion variable,
they can be cast into series expansions with integer coefficients. We also give
several results showing that the unique analytical solution of Calabi-Yau ODEs,
and, more generally, Picard-Fuchs linear ODEs, with solutions of maximal
weights, are always diagonal of rational functions. Besides, in a more
enumerative combinatorics context, generating functions whose coefficients are
expressed in terms of nested sums of products of binomial terms can also be
shown to be diagonals of rational functions. We finally address the question of
the relations between the notion of integrality (series with integer
coefficients, or, more generally, globally bounded series) and the modularity
of ODEs.Comment: This paper is the short version of the larger (100 pages) version,
available as arXiv:1211.6031 , where all the detailed proofs are given and
where a much larger set of examples is displaye
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