Gábor Hetyei

The topology of the independence complex

We introduce a large self-dual class of simplicial complexes for which we show that each member complex is contractible or homotopy equivalent to a sphere. Examples of complexes in this class include independence and dominance complexes of forests, pointed simplicial complexes, and their combinatorial Alexander duals.

Linear Inequalities for Flags in Graded Partially Ordered Sets

The closure of the convex cone generated by all flag f-vectors of graded partially ordered sets is shown to be polyhedral. In particular, we give the facet inequalities to the polar cone of all nonnegative chain-enumeration functionals on this class of partially ordered sets. These are in one-to-one correspondence with antichains of intervals on the set of ranks and thus are counted by Catalan numbers. Furthermore, we prove that the convolution operation introduced by Kalai assigns extreme rays to pairs of extreme rays in most cases. We describe the strongest possible inequalities for graded partially ordered sets of rank at most 5.

Flag Vectors of Eulerian Partially Ordered Sets

The closed cone of flag vectors of Eulerian partially ordered sets is studied. A new family of linear inequalities valid for Eulerian flag vectors is given. Half-Eulerian posets are defined. Certain limit posets of Billera and Hetyei are half-Eulerian; they give rise to extreme rays of the cone for Eulerian posets. Other extreme posets are formed from consideration of the cd -index. The cone of Eulerian flag vectors is completely determined up through rank seven.

Permutation Trees and Variation Statistics

In this paper we exploit binary tree representations of permutations to give a combinatorial proof of Purtills result 8 that???Anvcd(?)?c=a+bd=ab+ba=???Snvab(?),where Anis the set of Andre permutations,vcd(?) is thecd-statistic of an Andre permutation andvab(?) is theab-statistic of a permutation. Using Purtills proof as a motivation we introduce a new `Foata?Strehl-like? action on permutations. This Z2n?1-action allows us to give an elementary proof of Purtills theorem, and a bijection between Andre permutations of the first kind and alternating permutations starting with a descent. A modified version of our group action leads to a new class of Andre-like permutations with structure similar to that of simsun permutations.

On thecd-variation polynomials of André and simsun permutations

We prove a conjecture of Stanley on thecd-index of the semisuspension of the face poset of a simplicial shelling component. We give a new signed generalization of André permutations, together with a new notion ofcd-variation for signed permutations. This generalization not only allows us to compute thecd-index of the face poset of a cube, but also occurs as a natural set of orbit representatives for a signed generalization of the Foata-Strehl commutative group action on the symmetric group. From the induction techniques used, it becomes clear that there is more than one way to define classes of permutations andcd-variation such that they allow us to compute thecd-index of the same poset.

Decompositions of Partially Ordered Sets

Generalizing the proof of the theorem describing the closed cone of flag f-vectors of arbitrary graded posets, we give a description of the cone of flag f-vectors of planar graded posets. The labeling used is a special case of a “chain-edge labeling with the first atom property”, or FA-labeling, which also generalizes the notion of lexicographic shelling, or CL-labeling. The resulting analogy suggests a planar analogue of the flag h-vector. For planar Cohen–Macaulay posets the two h-vectors turn out to be equal, and the cone of flag h-vectors an orthant, whose dimension is a Fibonacci number. The use of FA-labelings also yields a simple enumeration of the facets in the order complex of an arbitrary graded poset such that the intersection of each cell with the previously attached cells is homotopic to a ball or to a sphere.

On the Stanley ring of cubical complex

We investigate the properties of the Stanley ring of a cubical complex, a cubical analogue of the Stanley-Reisner ring of a simplicial complex. We compute its Hilbert series in terms of thef-vector, and prove that by taking the initial ideal of the defining relations, with respect to the reverse lexicographic order, we obtain the defining relations of the Stanley-Reisner ring of the triangulation via “pulling the vertices” of the cubical complex. Applying an old idea of Hochster we see that this ring is Cohen-Macaulay when the complex is shellable, and we show that its defining ideal is generated by quadrics when the complex is also a subcomplex of the boundary complex of a convex cubical polytope. We present a cubical analogue of balanced Cohen-Macaulay simplicial complexes: the class of edge-orientable shellable cubical complexes. Using Stanleys results about balanced Cohen-Macaulay simplicial complexes and the degree two homogeneous generating system of the defining ideal, we obtain an infinite set of examples for a conjecture of Eisenbud, Green, and Harris. This conjecture says that theh-vector of a polynomial ring inn variables modulo an ideal which has ann-element homogeneous system of parameters of degree two, is thef-vector of a simplicial complex.

On the diameter of random Cayley graphs of the symmetric group

Let σ, π be two permutations selected at random from the uniform distribution on the symmetric group S n . By a result of Dixon [5], the subgroup G generated by σ, π is almost always (i.e. with probability approaching 1 as n → ∞) either S n or the alternating group A n . We prove that the diameter of the Cayley graph of G defined by {σ, π} is almost always not greater than exp ((½ + o (l)). (In n ) 2 ).

Relative tutte polynomials for coloured graphs and virtual knot theory

We introduce the concept of a relative Tutte polynomial of coloured graphs. We show that this relative Tutte polynomial can be computed in a way similar to the classical spanning tree expansion used by Tutte in his original paper on this subject. We then apply the relative Tutte polynomial to virtual knot theory. More specifically, we show that the Kauffman bracket polynomial (and hence the Jones polynomial) of a virtual knot can be computed from the relative Tutte polynomial of its face (Tait) graph with some suitable variable substitutions. Our method offers an alternative to the ribbon graph approach, using the face graph obtained from the virtual link diagram directly.

Generalizations of Eulerian partially ordered sets, flag numbers, and the Möbius function

A partially ordered set is r-thick if every nonempty open interval contains at least r elements. This paper studies the flag vectors of graded, r-thick posets and shows the smallest convex cone containing them is isomorphic to the cone of flag vectors of all graded posets. It also defines a k-analogue of the Mobius function and k-Eulerian posets, which are 2k-thick. Several characterizations of k-Eulerian posets are given. The generalized Dehn-Sommerville equations are proved for flag vectors of k-Eulerian posets. A new inequality is proved to be valid and sharp for rank 8 Eulerian posets.