997 research outputs found
On two unimodal descent polynomials
The descent polynomials of separable permutations and derangements are both
demonstrated to be unimodal. Moreover, we prove that the -coefficients
of the first are positive with an interpretation parallel to the classical
Eulerian polynomial, while the second is spiral, a property stronger than
unimodality. Furthermore, we conjecture that they are both real-rooted.Comment: 16 pages, 4 figure
Applications of the Brauer complex: card shuffling, permutation statistics, and dynamical systems
By algebraic group theory, there is a map from the semisimple conjugacy
classes of a finite group of Lie type to the conjugacy classes of the Weyl
group. Picking a semisimple class uniformly at random yields a probability
measure on conjugacy classes of the Weyl group. Using the Brauer complex, it is
proved that this measure agrees with a second measure on conjugacy classes of
the Weyl group induced by a construction of Cellini using the affine Weyl
group. Formulas for Cellini's measure in type are found. This leads to new
models of card shuffling and has interesting combinatorial and number theoretic
consequences. An analysis of type C gives another solution to a problem of
Rogers in dynamical systems: the enumeration of unimodal permutations by cycle
structure. The proof uses the factorization theory of palindromic polynomials
over finite fields. Contact is made with symmetric function theory.Comment: One change: we fix a typo in definition of f(m,k,i,d) on page 1
The Eulerian distribution on the involutions of the hyperoctahedral group is unimodal
The Eulerian distribution on the involutions of the symmetric group is
unimodal, as shown by Guo and Zeng. In this paper we prove that the Eulerian
distribution on the involutions of the hyperoctahedral group, when viewed as a
colored permutation group, is unimodal in a similar way and we compute its
generating function, using signed quasisymmetric functions.Comment: 11 pages, zero figure
Affine descents and the Steinberg torus
Let be an irreducible affine Weyl group with Coxeter complex
, where denotes the associated finite Weyl group and the
translation subgroup. The Steinberg torus is the Boolean cell complex obtained
by taking the quotient of by the lattice . We show that the
ordinary and flag -polynomials of the Steinberg torus (with the empty face
deleted) are generating functions over for a descent-like statistic first
studied by Cellini. We also show that the ordinary -polynomial has a
nonnegative -vector, and hence, symmetric and unimodal coefficients. In
the classical cases, we also provide expansions, identities, and generating
functions for the -polynomials of Steinberg tori.Comment: 24 pages, 2 figure
The Eulerian Distribution on Involutions is Indeed Unimodal
Let I_{n,k} (resp. J_{n,k}) be the number of involutions (resp. fixed-point
free involutions) of {1,...,n} with k descents. Motivated by Brenti's
conjecture which states that the sequence I_{n,0}, I_{n,1},..., I_{n,n-1} is
log-concave, we prove that the two sequences I_{n,k} and J_{2n,k} are unimodal
in k, for all n. Furthermore, we conjecture that there are nonnegative integers
a_{n,k} such that This statement is stronger than
the unimodality of I_{n,k} but is also interesting in its own right.Comment: 12 pages, minor changes, to appear in J. Combin. Theory Ser.
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