Coproducts and the cd-Index

The linear span of isomorphism classes of posets, P, has a Newtonian coalgebra structure. We observe that the ab-index is a Newtonian coalgebra map from the vector space P to the algebra of polynomials in the noncommutative variables a and b. This enables us to obtain explicit formulas showing how the cd-index of the face lattice of a convex polytope changes when taking the pyramid and the prism of the polytope and the corresponding operations on posets. As a corollary, we have new recursion formulas for the cd-index of the Boolean algebra and the cubical lattice. Moreover, these operations also have interpretations for certain classes of permutations, including simsun and signed simsun permutations. We prove an identity for the shelling components of the simplex. Lastly, we show how to compute the ab-index of the Cartesian product of two posets given the ab-indexes of each poset.     

格点偏序集的一些组合恒等式（英）

This paper is devoted to the counting of j-chains of lattice-point poset. The incidence function and two basic enumerators associated with it are introduced and evaluated. Our results contain a lot of combinatorial sums.     

Compact Compatible Topologies for Posets and Graphs

A topology on the vertex set of a comparability graph G is said to be compatible (respectively, weakly compatible) with G if each induced subgraph (respectively, each finite induced subgraph) is topologically connected if and only it it is graph-connected; a weakly compatible topology on the vertex set of a graph completely determines the graph structure. We consider here the problem of deciding whether or not a comparability graph has a compact compatible or weakly compatible topology and in the case of graphs with small cycles, hence in the case of trees, we give a characterization.     

Polygon Posets and the Weak Order of Coxeter Groups

We explore the connection between polygon posets, which is a class of ranked posets with an edge-labeling which satisfies certain polygon properties, and the weak order of Coxeter groups. We show that every polygon poset is isomorphic to a join ideal in the weak order, and for Coxeter groups where no pair of generators have infinite order the converse is also true.The class of polygon posets is seen to include the class of generalized quotients defined by Björner and Wachs, while itself being included in the class of alternative generalized quotients also considered by these authors. By studying polygon posets we are then able to answer an open question about common properties of these two classes.     

5元素集合上T_0拓扑总数的计算

利用有限偏序集上的几个重要结果并借助于拓扑空间对应的特殊化序与拓扑之间的关系计算得出5元素集合上T0拓扑总数为4231,拓扑总数为6942.     

Ortho and Causal Closure Operations in Ordered Vector Spaces

On a non-trivial partially ordered real vector space V the orthogonality relation is defined by incomparability and is a complete lattice of double orthoclosed sets. In an earlier paper we defined an integrally open ordered vector space V and proved orthomodularity of . We shall say that is an orthogonal set when for all with , we have . We consider two different closure operations and (ortho and causal closure) and prove: V is integrally open iff for every orthogonal set . Hence follows: if V is integrally open, then . Received July 6, 2007; accepted in final form July 31, 2007.     

Deletion-restriction for subspace arrangements

Let A be a subspace arrangement in V with a designated maximal affine subspace A₀. Let A^′=A?{A₀} be the deletion of A₀ from A and A^″={A∩A₀|A∩A₀≠∅} be the restriction of A to A₀. Let M=V?_?A∈AA be the complement of A in V. If A is an arrangement of complex affine hyperplanes, then there is a split short exact sequence, 0→Hk(M^′)→Hk(M)→Hk+1−codimR(A₀)(M^″)→0. In this paper, we determine conditions for when the triple (A,A^′,A^″) of arrangements of affine subspaces yields the above split short exact sequence. We then generalize the no-broken-circuit basis nbc of Hk(M) for hyperplane arrangements to deletion-restriction subspace arrangements.     

Linear Discrepancy of the Product of Two Chains

The linear discrepancy of a partially ordered set P = (X, ≺) is the minimum integer l such that ∣f(a) − f(b)∣ ≤ l for any injective isotone and any pair of incomparable elements a, b in X. It measures the degree of difference of P from a chain. Despite of increasing demands to the applications, the discrepancies of just few simple partially ordered sets are known. In this paper, we obtain the linear discrepancy of the product of two chains. For this, we firstly give a lower bound of the linear discrepancy and then we construct injective isotones on the product of two chains, which show that the obtained lower bound is tight.     

Decision Effects,the Hahn–Jordan Decomposition for States,and Their Connection to Ludwig’s Axiomatic Approach to Quantum Mechanics

We begin with a quantum logic carrying a large collection of states. We then form a dual pair of Banach spaces—base normed and order unit normed—containing the states and the logic, respectively. A Galois connection on the face lattices of the states and the dual positive order unit interval is introduced. The elements of the logic are connected to a dense subset of the extreme points of this order interval in the order unit space using a generalized form of the Hahn–Jordan decomposition theorem. Decision effects are defined and identified with the elements of the original logic. Finally, an important axiom of Ludwig is introduced which ties together all the lattices of Galois closed faces of states, Galois closed order intervals of the positive order unit interval, decision effects, and the original quantum logic. The emphasis here is on the consequences of functional analytic assumptions. The paper concludes with a simple example where Ludwig’s axiom does not hold and we see parts of the theory dissolve. PACS: 02.30.Sa, 03.65.Ta.     

拟C-偏序集的若干性质

Menon在对连续Domain进行推广时引入C-偏序集的概念,即可用主滤子与上完备下集分离点的偏序集。基于Menon的思想,我们把拟连续偏序集推广至拟C-偏序集,即可用有限生成上集与上完备下集分离点的偏序集。结果表明,C-偏序集、拟连续偏序集都为拟C-偏序集,反之则不一定成立,并且,拟C-偏序集及其基具有类似于C-偏序集的关于映射、乘积等的封闭性。