We prove the equivalence of EL-shellability and existence recursive atom ordering independent roots. show that a comodernistic lattice, as defined by Schweig Woodroofe, admits roots, therefore is EL-shellable. also present discuss simpler EL-shelling on one most important classes order congruence lattices.

we consider a class of hypergraphs called hypercycles and we show that a hypercycle $c_n^{d,alpha}$ is shellable or sequentially the cohen--macaulay if and only if $nin{3,5}$. also, we characterize cohen--macaulay hypercycles. these results are hypergraph versions of results proved for cycles in graphs.

We prove that if a simplicial complex ∆ is shellable, then the intersection lattice L∆ for the corresponding diagonal arrangement A∆ is homotopy equivalent to a wedge of spheres. Furthermore, we describe precisely the spheres in the wedge, based on the data of shelling. Also, we give some examples of diagonal arrangements A where the complement MA is K(π, 1), coming from rank 3 matroids.

In this paper we study the partially ordered set of the involutions of the symmetric group Sn with the order induced by the Bruhat order of Sn . We prove that this is a graded poset, with rank function given by the average of the number of inversions and the number of excedances, and that it is lexicographically shellable, hence Cohen-Macaulay, and Eulerian.

We show that Ding's partial order on maximal rook placements on any Ferrers board has a symmetric chain decomposition and is EL-shellable. As a consequence the partial order is Peck, and we show that it has Mobius function values of 1; 0 or +1.

The h-polynomial of the barycentric subdivision any n-dimensional cubical complex with nonnegative h-vector is shown to have only real roots and be interlaced by Eulerian polynomial type Bn. This result applies subdivisions shellable complexes and, in particular, convex polytopes answers affirmatively a question Brenti, Mohammadi Welker.

We consider a class of hypergraphs called hypercycles and we show that a hypercycle $C_n^{d,alpha}$ is shellable or sequentially the Cohen--Macaulay if and only if $nin{3,5}$. Also, we characterize Cohen--Macaulay hypercycles. These results are hypergraph versions of results proved for cycles in graphs.