Classification of the factorial functions of Eulerian binomial and Sheffer posets

Abstract

We give a complete classification of the factorial functions of Eulerian binomial posets. The factorial function B(n) either coincides with \(n!\), the factorial function of the infinite Boolean algebra, or \(2^{n-1}\), the factorial function of the infinite butterfly poset. We also classify the factorial functions for Eulerian Sheffer posets. An Eulerian Sheffer poset with binomial factorial function \(B(n) = n!\) has Sheffer factorial function D(n) identical to that of the infinite Boolean algebra, the infinite Boolean algebra with two new coatoms inserted, or the infinite cubical poset. Moreover, we are able to classify the Sheffer factorial functions of Eulerian Sheffer posets with binomial factorial function \(B(n) = 2^{n-1}\) as the doubling of an upside down tree with ranks 1 and 2 modified. When we impose the further condition that a given Eulerian binomial or Eulerian Sheffer poset is a lattice, this forces the poset to be the infinite Boolean algebra \(B_X\) or the infinite cubical lattice \(C_X^{< \infty}\). We also include several poset constructions that have the same factorial functions as the infinite cubical poset, demonstrating that classifying Eulerian Sheffer posets is a difficult problem.