Upho lattices II: ways of realizing a core

A poset is called upper homogeneous, or “upho,” if all of its principal order filters are isomorphic to the whole poset. In previous work of the first author, it was shown that each (finite-type N-graded) upho lattice has associated to it a finite graded lattice, called its core, which determines the rank generating function of the upho lattice. In that prior work the question of which finite graded lattices arise as cores was explored. Here, we study the question of in how many different ways a given finite graded lattice can be realized as the core of an upho lattice. We show that if the finite lattice has no nontrivial automorphisms, then it is the core of finitely many upho lattices. We also show that the number of ways a finite lattice can be realized as a core is unbounded, even when restricting to rank-two lattices. We end with a discussion of a potential algorithm for listing all the ways to realize a given finite lattice as a core.

💡 Research Summary

The paper investigates the relationship between upper‑homogeneous (upho) lattices and their finite graded “cores,” focusing on how many distinct upho lattices can share a given core. For a finite graded lattice L, the authors define κ(L) to be the number of non‑isomorphic upho lattices whose core is L. The study addresses two fundamental questions: (1) When is κ(L) finite? (2) How large can κ(L) become?

The authors first recall that an upho poset is one in which every principal filter Vₚ is isomorphic to the whole poset. In the finite‑type N‑graded setting, each upho lattice L has a core defined as the interval