Schnyder characterized planar graphs in terms of order dimension. This seminal result found several extensions. A particularly far reaching extension is the Brightwell-Trotter Theorem about planar maps. It states that the order dimension of the incidence poset PM of vertices, edges and faces of a planar map M has dimension at most 4. The original proof generalizes the machinery of Schnyder-path...

To an extended generalized permutohedron we associate the weighted integer points enumerator, whose principal specialization is f-polynomial. In case of poset cones it refines Gessel?s P-partitions enumerator. We show that this enumerator a quasisymmetric function obtained by universal morphism from Hopf algebra posets.

We explore the problem of constructing maximal and unbounded filters on computable posets. We obtain both computability results and reverse mathematics results. A maximal filter is one that does not extend to a larger filter. We show that every computable poset has a ∆2 maximal filter, and there is a computable poset with no Π1 or Σ 0 1 maximal filter. There is a computable poset on which every...

A partially ordered set (poset) is planar if it has a planar Hasse diagram. The dimension of a bounded planar poset is at most two. We show that the dimension of a planar poset having a greatest lower bound is at most three. We also construct four-dimensional planar posets, but no planar poset with dimension larger than four is known. A poset is called a tree if its Hasse diagram is a tree in t...

Let U0, U1 be two normal measures on κ. We say that U0 is in the Mitchell ordering less than U1, U0 C U1, if U0 ∈ Ult(V,U1). The relation is well-known to be transitive and well-founded. It has been an open problem to find a model where C embeds the four-element poset . We find a generic extension where all well-founded posets are embeddable. Hence there is no structural restriction on the Mitc...

The poset retraction problem for a poset P is whether a given poset Q containing P as a subposet admits a retraction onto P, that is, whether there is a homomorphism from Q onto P which fixes every element of P. We study this problem for finite series-parallel posets P. We present equivalent combinatorial, algebraic, and topological charaterisations of posets for which the problem is tractable,...