Maximal antichains of isomorphic subgraphs of the Rado graph

If ?R,E? is the Rado graph andR(R) the set of its copies inside R, then ?R(R), ?? is a chain-complete and non-atomic partial order of the size 2x0 . A family A ? R(R) is a maximal antichain in this partial order iff (1) A ? B does not contain a copy of R, for each different A, B ?A and (2) For each S ? R(R) there is A ? A such that A ? S contains a copy of R. We show that the partial order ?R(R), ?? contains maximal antichains of size 2x0, X0 and n, for each positive integer n (thus, of all possible cardinalities, under CH). The results are compared with the corresponding known results concerning the partial order ?[?]?, ??.

Sperner's Problem for G-Independent Families

Given a graph G, let Q(G) denote the collection of all independent (edge-free) sets of vertices in G. We consider the problem of determining the size of a largest antichain in Q(G). When G is the edgeless graph, this problem is resolved by Sperner's theorem. In this paper, we focus on the case where G is the path of length n − 1, proving that the size of a maximal antichain is of the same order as the size of a largest layer of Q(G).

Another Note on Dilworth's Decomposition Theorem

This paper proposes a new proof of Dilworth's theorem. The proof is based upon the minflow/maxcut property in flow networks. In relation to this proof, a new method to find both a Dilworth decomposition and a maximal antichain is presented.

Splitting Numbers of Grids

For a subset $S$ of a finite ordered set $P$, let $$S\!\uparrow\;=\{x\in P:x\ge s \hbox{ for some }s\in S\}\quad\hbox{and} \quad S\!\downarrow\;=\{x\in P:x\le s \hbox{ for some }s\in S\}.$$ For a maximal antichain $A$ of $P$, let $$s(A)=\max_{A=U\cup D}{|U\!\uparrow|+|D\!\downarrow|\over|P|}\ ,$$ the maximum taken over all partitions $U\cup D$ of $A$, and $$s_k(P)=\min_{A\in {\cal A}(P),|A|=k}s(A)$$ where we assume $P$ contains at least one maximal antichain of $k$ elements. Finally, for a class ${\cal C}$ of finite ordered sets, we define $$s_k({\cal C})=\inf_{P\in {\cal C}}s_k(P).$$ Thus $s_k({\cal C})$ is the greatest proportion $r$ satisfying: every $k$-element maximal antichain of a member $P$ of ${\cal C}$ can be "split" into sets $U$ and $D$ so that $U\!\uparrow\cup\; D\!\downarrow$ contains at least $r|P|$ elements. In this paper we determine $s_k({\cal G}_k)$ for all $k\ge 1$, where ${\cal G}_k=\{{\bf k}\times{\bf n}:n\ge k\}$ is the family of all $k$ by $n$ "grids".

Minimum sized fibres in distributive lattices

A subset F of an ordered set X is a fibre of X if F intersects every maximal antichain of X. We find a lower bound on the function ƒ (D), the minimum fibre size in the distributive lattice D, in terms of the size of D. In particular, we prove that there is a constant c such that In the process we show that minimum fibre size is a monotone property for a certain class of distributive lattices. This fact depends upon being able to split every maximal antichain of this class of distributive lattices into two parts so that the lattice is the union of the upset of one part and the downset of the other.

Possible behaviours of the reflection ordering of stationary sets

If S, T are stationary subsets of a regular uncountable cardinal κ, we say that S reflects fully in T, S < T, if for almost all α ∈ T (except a nonstationary set) S ∩ α stationary in α. This relation is known to be a well-founded partial ordering. We say that a given poset P is realized by the reflection ordering if there is a maximal antichain 〈Xp: p ∈ P〉 of stationary subsets of Reg(κ) so thatWe prove that if , and P is an arbitrary well-founded poset of cardinality ≤ κ+ then there is a generic extension where P is realized by the reflection ordering on κ.